Correct WinRT exclusion to top-level pro file

So far only the module was excluded, but not the examples.
Adopt to a style which has been approved for other modules as well
by using requires.

Change-Id: I8a8bbeb8c5db1bdb2e508da0988a38bbb1789986
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>