[ios] Fixed designable warning when showing user location

Turn the Shows User Location inspectable into a no-op when running in the designables agent. Otherwise, we complain that the agent lacks Location Services permissions.

platform/ios/src/MGLMapView.mm

@@ -2999,7 +2999,7 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
 
 - (void)setShowsUserLocation:(BOOL)showsUserLocation
 {
-    if (showsUserLocation == _showsUserLocation) return;
+    if (showsUserLocation == _showsUserLocation || _isTargetingInterfaceBuilder) return;
 
     _showsUserLocation = showsUserLocation;