Qt5WebKitWidgets should keep private dependencies private

The qmake file webkit_modules was not loaded, which meant we never
separated private Qt dependencies from public Qt dependencies in the QT
qmake variable.

Task-number: QTBUG-39249
Change-Id: Iadad6b652cee5a51894c4ac2e9979d7d41619d27
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>