Gawain: fix compiler warnings

vertex_format.c implements both public and private functions, so needs to include the private header.

Follow-up to 7c57ca5109

intern/gawain/src/vertex_format.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 "vertex_format.h"
+#include "vertex_format_private.h"
 #include <stddef.h>
 #include <string.h>