Use correct enum in wxOSX wxMediaCtrl code

The values we check for are elements of AVPlayerItemStatus enum and not
AVPlayerStatus, so use the correct type for the variable.

This actually doesn't change anything and the enum elements have the
same values anyhow, but the code makes more sense now and avoids
-Wenum-compare-switch warnings from Xcode 10 compiler.

src/osx/cocoa/mediactrl.mm

@@ -162,7 +162,7 @@ private:
         id val = [change objectForKey:NSKeyValueChangeNewKey];
         if ( val != [NSNull null ] )
         {
-            AVPlayerStatus status = (AVPlayerStatus) [ val integerValue];
+            AVPlayerItemStatus status = (AVPlayerItemStatus) [ val integerValue];
 
             switch (status)
             {