Port mod-test to --enable-gcc-warnings

* test/data/emacs-module/mod-test.c (NDEBUG): Undef, to prevent
GCC from complaining “error: unused variable ‘dummy’” and failing
to build the test, if --enable-gcc-warnings.

test/data/emacs-module/mod-test.c

@@ -19,7 +19,9 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
 #include "config.h"
 
+#undef NDEBUG
 #include <assert.h>
+
 #include <errno.h>
 #include <limits.h>
 #include <stdio.h>