turn on -Werror and fix resulting errors

src/modules/fixedwing_backside/fixedwing_backside_main.cpp

@@ -115,7 +115,7 @@ int fixedwing_backside_main(int argc, char *argv[])
 					 SCHED_PRIORITY_MAX - 10,
 					 5120,
 					 control_demo_thread_main,
-					 (argv) ? (const char **)&argv[2] : (const char **)NULL);
+					 (argv) ? (char * const *)&argv[2] : (char * const *)NULL);
 		exit(0);
 	}