The current version of qtwebkit used in qt5 does not work with Qt/QPA.
The problem is in the netscape plugin API's usage of QX11EmbedContainer.
Disable the netscape plugin API until qtwebkit in qt5 can be updated to
a fixed version.

Change-Id: I833bfff2f1b1059810206b4160494eb59734b5ea
Reviewed-on: http://codereview.qt.nokia.com/1337


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>

This feature does not currently work with webkit's custom build script,
so disable it.