fix gettid type on Ubuntu 20.04

[karnauskas]

Fix for conflicting data type

In file included from /home/mk/code/USBProxy-legacy/src/lib/RelayWriter.cpp:14:
/home/mk/code/USBProxy-legacy/src/lib/get_tid.h:12:10: error: conflicting declaration of C function ‘long int gettid()’
   12 | long int gettid();
      |          ^~~~~~
In file included from /usr/include/unistd.h:1170,
                 from /home/mk/code/USBProxy-legacy/src/lib/RelayWriter.cpp:7:
/usr/include/x86_64-linux-gnu/bits/unistd_ext.h:34:16: note: previous declaration ‘__pid_t gettid()’
   34 | extern __pid_t gettid (void) __THROW;
      |                ^~~~~~

[dominicgs]

Thanks!