turn on -Werror and fix resulting errors

src/drivers/ardrone_interface/ardrone_interface.c

@@ -121,7 +121,7 @@ int ardrone_interface_main(int argc, char *argv[])
 						    SCHED_PRIORITY_MAX - 15,
 						    1100,
 						    ardrone_interface_thread_main,
-						    (argv) ? (const char **)&argv[2] : (const char **)NULL);
+						    (argv) ? (char * const *)&argv[2] : (char * const *)NULL);
 		exit(0);
 	}