winrtrunner is needed for running auto tests on winrt. As the tool
resides in qttools and not qtbase and it not changed often, it was
decided to handle it similar to jom and see it as an external tool.
Change-Id: I9636962df51f98bef6aad3e340c565f50e393949
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>