adds missing freetype2/ include path for gcc build

[marcidy]

Building with gcc fails due to missing the freetype2 include path in build.sh, which is already available in the clang_common var.

This fix adds "-I/usr/include/freetype2to thegcc_common` var, enabled a successful build.

The error is:

$ build.sh gcc raddbg
...
In file included from ../src/eval/eval_inc.h:7,
                 from ../src/raddbg/raddbg_main.c:254:
../src/eval/eval_core.h:1215:1: warning: multi-line comment [-Wcomment]
 1215 | //        /            /             |                     \
      | ^
In file included from ../src/font_provider/font_provider_inc.h:35,
                 from ../src/raddbg/raddbg_main.c:257:
../src/font_provider/freetype/font_provider_freetype.h:11:10: fatal error: ft2build.h: No such file or dir
   11 | #include <ft2build.h>
      |          ^~~~~~~~~~~~
compilation terminated.

With the fix, the build is successful.