_makesema declaration was giving a warning when compiling with C:

	In file included from golang/runtime/libgolang_test_c.c:26:
	./golang/libgolang.h:237:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
	 LIBGOLANG_API _sema *_makesema();
	 ^~~~~~~~~~~~~

Fix the prototype to be valid for both C and C++.
Fixes: 34b7a1f4.