Compilation on mac with latest qt5 fails with

   fatal error: 'QWebSettings' file not found

The compiler call adds "include/QtWebKitWidgets" to include path,
not "include/QtWebKit" . This might be a qmake related issue on mac.
Attempt to fix it by adding an explicit webkit dependency.

Change-Id: I032dd4dda7371a8d477f8544b0ba0004856f3d76
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>