[Qt] Doesn't build with QtMultimedia
https://bugs.webkit.org/show_bug.cgi?id=111847

Reviewed by Jocelyn Turcotte.

Fix include.

* platform/graphics/qt/MediaPlayerPrivateQt.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145210 268f45cc-cd09-0410-ab3c-d52691b4dbfc