turn on -Werror and fix resulting errors

src/examples/fixedwing_control/main.c

@@ -423,7 +423,7 @@ int ex_fixedwing_control_main(int argc, char *argv[])
 					 SCHED_PRIORITY_MAX - 20,
 					 2048,
 					 fixedwing_control_thread_main,
-					 (argv) ? (const char **)&argv[2] : (const char **)NULL);
+					 (argv) ? (char * const *)&argv[2] : (char * const *)NULL);
 		thread_running = true;
 		exit(0);
 	}

src/examples/fixedwing_control/module.mk

@@ -41,3 +41,5 @@ SRCS		= main.c \
 		  params.c
 
 MODULE_STACKSIZE = 1200
+
+EXTRACFLAGS = -Wno-error