Include string.h for memset() on OpenBSD

platform/x11/os_x11.cpp

@@ -33,6 +33,7 @@
 #include "key_mapping_x11.h"
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include "print_string.h"
 #include "servers/physics/physics_server_sw.h"