Fix build with libstdc++

libstdc++ doesn't include <cstring> inside of <string>, unlike libc++.

src/objdef.cpp

@@ -17,9 +17,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
+#include <strings.h>
 #include "objdef.h"
 #include "util/numeric.h"
 #include "debug.h"
+#include "porting.h"
 #include "log.h"
 #include "gamedef.h"