Declare `emscriptenloop` as `static`

It's not going to link with anything in a different translation unit, so
just to make sure, we declare it as static.

desktop_version/src/main.cpp

@@ -355,7 +355,7 @@ static void inline deltaloop(void);
 static void cleanup(void);
 
 #ifdef __EMSCRIPTEN__
-void emscriptenloop(void)
+static void emscriptenloop(void)
 {
     timePrev = time_;
     time_ = SDL_GetTicks();