N3DS: Set keyboard focus to newly created windows.

This fixes polling issues with Joystick subsystem where
`SDL_PrivateJoystickShouldIgnoreEvent` would always return true, thus
ignoring all inputs.

src/video/n3ds/SDL_n3dsvideo.c

@@ -178,6 +178,7 @@ N3DS_CreateWindow(_THIS, SDL_Window *window)
     display_data = (DisplayDriverData *) SDL_GetDisplayDriverData(window->display_index);
     window_data->screen = display_data->screen;
     window->driverdata = window_data;
+    SDL_SetKeyboardFocus(window);
     return 0;
 }