lang/gnat15: remove -Werror

Compilation fails on arm64 because
enable-execute-stack.c:33:13: error: 'check_enabling' declared 'static' but never defined [-Werror=unused-function]

33 | static void check_enabling (void) __attribute__ ((unused));
   |             ^~~~~~~~~~~~~~

cc1: all warnings being treated as errors

PR: 292708
Reported by: pkg-fallout