Disable warning about QObject::event being hidden

Found by Clang 3.6:
qdeclarativemediametadata_p.h:292:14: warning: 'QDeclarativeMediaMetaData::event' hides overloaded virtual function [-Woverloaded-virtual]
qobject.h:116:18: note: hidden overloaded virtual function 'QObject::event' declared here: different number of parameters (1 vs 0)

Change-Id: I4324c984649738c9625cc3eb9dddea6e2abf46a9
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>