Fix Makefile issue

[williamthome]

This PR fixes an issue using bcrypt in a GitHub Action/Docker Container.

This issue describes the error:

make[1]: Entering directory '/tmp/zotonic/_build/default/lib/bcrypt/c_src'
Error! Failed to eval: io:format("~s/erts-~s/include/", [code:root_dir(), erlang:system_info(version)]).

Error! Failed to eval: io:format("~s", [code:lib_dir(erl_interface, include)]).

cc -O3 -std=c99 -finline-functions -Wall -Wmissing-prototypes -D_DEFAULT_SOURCE -fPIC -I  -I   -c -o bcrypt_nif.o bcrypt_nif.c
bcrypt_nif.c:23:10: fatal error: erl_nif.h: No such file or directory
   23 | #include "erl_nif.h"
      |          ^~~~~~~~~~~
compilation terminated.
make[1]: *** [Makefile:77: bcrypt_nif.o] Error 1
make[1]: Leaving directory '/tmp/zotonic/_build/default/lib/bcrypt/c_src'
===> Hook for compile failed!

make: *** [GNUmakefile:37: compile] Error 1
Error: Process completed with exit code 2.

Using the changes of this PR, no error is raised. Those changes are based on this issue.

[mworrell]

@williamthome Thanks! I've released v1.2.2 to Hex

[williamthome]

Perfect! Thank you!