Enable virtual joystick support for Emscripten

include/build_config/SDL_build_config_emscripten.h

@@ -163,6 +163,7 @@
 
 /* Enable various input drivers */
 #define SDL_JOYSTICK_EMSCRIPTEN 1
+#define SDL_JOYSTICK_VIRTUAL    1
 
 /* Enable various sensor drivers */
 #define SDL_SENSOR_DUMMY 1