Make sure CallbackStream doesn't get destroyed before the Music

Music's destructor call's stop(), which in turn calls onSeek.
For CSFML, this requires the CallbackStream to stay alive until that point.

In C++, destructors for fields are called in reverse declaration order.
We swap the order of the fields, putting the Music after the
CallbackStream, so it gets destroyed before it.

src/SFML/Audio/MusicStruct.h

@@ -37,8 +37,8 @@
 ////////////////////////////////////////////////////////////
 struct sfMusic
 {
-    sf::Music This;
     CallbackStream Stream;
+    sf::Music This;
 };