gltfpack: Fix libc++ compilation without transitive includes

stdlib.h is needed for free() which is otherwise absent when using
libc++ and _LIBCPP_REMOVE_TRANSITIVE_INCLUDES.

Author: zeux

gltf/image.cpp

@@ -1,6 +1,7 @@
 // This file is part of gltfpack; see gltfpack.h for version/license details
 #include "gltfpack.h"
 
+#include <stdlib.h>
 #include <string.h>
 
 static const char* kMimeTypes[][2] = {