libgccjit.h: Uncomment macro definition for testing gcc_jit_context_new_bitcast support

The macro definition for LIBGCCJIT_HAVE_gcc_jit_context_new_bitcast
was earlier located in the documentation comment for
gcc_jit_context_new_bitcast, making it unavailable to code that
consumed libgccjit.h. This commit moves the definition out of the
comment, making it effective.

gcc/jit/ChangeLog:
	* libgccjit.h (LIBGCCJIT_HAVE_gcc_jit_context_new_bitcast): Move
	definition out of comment.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>