Added missing px4_ prefixes

NuttX build required missing px4_ prefix for systemreset and task_spawn_cmd

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/systemcmds/tests/test_mount.c

@@ -288,7 +288,7 @@ test_mount(int argc, char *argv[])
 	fsync(fileno(stdout));
 	fsync(fileno(stderr));
 	usleep(50000);
-	systemreset(false);
+	px4_systemreset(false);
 
 	/* never going to get here */
 	return 0;