testgdk: Update icon filename for the new png image

VisualC-GDK/tests/testgdk/src/testgdk.cpp

@@ -339,7 +339,7 @@ static bool SDLCALL GDKEventWatch(void* userdata, SDL_Event* event)
 int main(int argc, char *argv[])
 {
     int i;
-    const char *icon = "icon.bmp";
+    const char *icon = "icon.png";
     char *soundname = NULL;
     bool suppressdraw = false;