Gawain: fix compilation on MSVC 2013

attrib_binding.c uses NULL, so include stddef.
Also swapped stdlib for stddef in vertex_format.c, since it only needs NULL.

Not sure why MSVC 2017 and clang/Mac work without this...

Thanks to @youle for reporting!

intern/gawain/src/attrib_binding.c

@@ -10,6 +10,7 @@
 // the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 #include "attrib_binding.h"
+#include <stddef.h>
 
 #if MAX_VERTEX_ATTRIBS != 16
   #error "attrib binding code assumes MAX_VERTEX_ATTRIBS = 16"

intern/gawain/src/vertex_format.c

@@ -10,7 +10,7 @@
 // the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 #include "vertex_format.h"
-#include <stdlib.h>
+#include <stddef.h>
 #include <string.h>
 
 #define PACK_DEBUG 0