Add typecast for C++11 compilation of OS X wxMediaCtrl

When compiling in C++11 mode the typecast of AVPlayerStatus in the callback is required to avoid a compiler error.

src/osx/cocoa/mediactrl.mm

@@ -578,7 +578,7 @@ private:
         id val = [change objectForKey:NSKeyValueChangeNewKey];
         if ( val != [NSNull null ] )
         {
-            AVPlayerStatus status = [ val integerValue];
+            AVPlayerStatus status = (AVPlayerStatus) [ val integerValue];
 
             switch (status)
             {