add 256px icon layer to mumble icon

* this was suggested by submitted patch 321.
This shouldn’t hurt, only increasing file-size a bit.
Although probably very rarely in use, the patch showed interest/usage of it.

src/tests/IconSync.cpp

@@ -25,6 +25,7 @@ int main(int argc, char **argv) {
 	sizes << 64;
 	sizes << 96;
 	sizes << 128;
+	sizes << 256;
 
 	QStringList qslImages;