Remove incorrect forward declaration

ANativeWindow is a struct, not a class, and the incorrect forward
declaration causes a warning when -isystem is removed from
system/core/include.

Test: m -j native
Bug: 31492149
Change-Id: I3516aa98defe1944a9776c07f52f4ee580366988
diff --git a/include/media/mediaplayer.h b/include/media/mediaplayer.h
index cec9d99..389ec01 100644
--- a/include/media/mediaplayer.h
+++ b/include/media/mediaplayer.h
@@ -30,7 +30,7 @@
 #include <utils/KeyedVector.h>
 #include <utils/String8.h>
 
-class ANativeWindow;
+struct ANativeWindow;
 
 namespace android {