MediaPlayerService: avoid invalid static cast

Bug: 30204103
Change-Id: Ie0dd3568a375f1e9fed8615ad3d85184bcc99028
diff --git a/media/libmediaplayerservice/MediaPlayerService.h b/media/libmediaplayerservice/MediaPlayerService.h
index 3b96e88..9ef2180 100644
--- a/media/libmediaplayerservice/MediaPlayerService.h
+++ b/media/libmediaplayerservice/MediaPlayerService.h
@@ -287,6 +287,7 @@
     virtual status_t            dump(int fd, const Vector<String16>& args);
 
             void                removeClient(wp<Client> client);
+            bool                hasClient(wp<Client> client);
 
     // For battery usage tracking purpose
     struct BatteryUsageInfo {