Fixed missing include breaking non-windows builds

Author: grahamboree

DebugUtils/Include/RecastDump.h

@@ -19,6 +19,8 @@
 #ifndef RECAST_DUMP_H
 #define RECAST_DUMP_H
 
+#include <stddef.h>
+
 class rcContext;
 struct rcCompactHeightfield;
 struct rcContourSet;