Fix config.tests for directshow backend.

directshow camera backend requires header qedit.h(currently removed since Windows SDK v7.0),
so if there is only Windows SDK v7.0, directshow backend should be disabled, since the directshow
mediaplayer will also be replaced by wmf backend.

Change-Id: I0de60fdcb683a57cc0aa3570f769179afc0c581b
Reviewed-by:Michael Goddard
(cherry picked from commit 68d78008f9d1eb82de673b8f4c8e73bdde2150c2)
Reviewed-on: http://codereview.qt-project.org/5494


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>