fix compile on debian buster

[fred0r]

tried to compile on current debian buster with latest premake5, but failed with:

user@host:~/work/AmxxCurl/build/gmake$ make                                                                                                                      
==== Building AmxxCurl (releasedll) ====                                                                                                                    
Creating obj/ReleaseDLL                                                                                                                                         
amx_curl_callback_class.cc                                                                                                                                      
asio_poller.cc                                                                                                                                                  
callbacks.cc                                                                                                                                                    
In file included from ../../src/amx_curl_manager_class.h:13,                                                                                                    
                 from ../../src/amx_curl_controller_class.h:4,                                                                                                  
                 from ../../src/callbacks.cc:3:                                                                                                              
../../src/amx_curl_class.h: In member function ‘void AmxCurl::Perform(const char*, int, cell*, int)’:                                                           
../../src/amx_curl_class.h:45:56: error: ‘bind’ is not a member of ‘std’                                                                                        
         CurlMulti::CurlPerformComplete callback = std::bind(&AmxCurl::OnPerformComplete, this, std::placeholders::_1);                                         
                                                        ^~~~                                                                                                    
../../src/amx_curl_class.h:45:56: note: ‘std::bind’ is defined in header ‘<functional>’; did you forget to ‘#include <functional>’?                             
../../src/amx_curl_class.h:6:1:                                                                                                                                  
+#include <functional>                                                                                                                                      

../../src/amx_curl_class.h:45:56:                                                                                                                               
         CurlMulti::CurlPerformComplete callback = std::bind(&AmxCurl::OnPerformComplete, this, std::placeholders::_1);                                         
                                                        ^~~~                                                                                                     
../../src/amx_curl_class.h:45:101: error: ‘std::placeholders’ has not been declared                                                                             
         CurlMulti::CurlPerformComplete callback = std::bind(&AmxCurl::OnPerformComplete, this, std::placeholders::_1);                                          
                                                                                                     ^~~~~~~~~~~~                                             
make[1]: *** [AmxxCurl.make:141: obj/ReleaseDLL/callbacks.o] Fehler 1                                                                                            
make: *** [Makefile:27: AmxxCurl] Fehler 2                                

same happend with curl_multi_class.cc, so simply added #include <functional> to both. module loads fine on amxx 1.10.5417.

[Polarhigh]

thank you for merge request, i will take a look soon