Fix not-quite-harmless warning in wxAVMediaBackend code

Take the "player" of the right type in -[wxAVView initWithFrame:player:]
method: we need a wxAVPlayer, which has a writable "playerLayer" field that we
set, and not an arbitrary AVPlayer here.

This fixes a warning which could become a problem if anything else than
wxAVPlayer was ever passed as parameter to this function.

src/osx/cocoa/mediactrl.mm

@@ -332,7 +332,7 @@ private:
     }
 }
 
-- (id) initWithFrame:(NSRect)rect player:(AVPlayer*) player
+- (id) initWithFrame:(NSRect)rect player:(wxAVPlayer*) player
 {
     if ( !(self=[super initWithFrame:rect]) )
         return nil;