Port --enable-gcc-warnings to Ubuntu 16.10

* src/gmalloc.c: Include <stdlib.h>, so it declares
hybrid_aligned_alloc (the definiens of the aligned_alloc macro),
so that GCC doesn't complain that hybrid_aligned_alloc is
defined without being declared.

src/gmalloc.c

@@ -26,6 +26,7 @@ License along with this library.  If not, see <http://www.gnu.org/licenses/>.
 #endif
 
 #include <stddef.h>
+#include <stdlib.h>
 #include <string.h>
 #include <limits.h>
 #include <stdint.h>