The WinRT positioning plugin can also be compiled for the desktop. This
support has been experimental and is now enabled by default via checking
existence of the API on the build machine.

[ChangeLog][Positioning][Windows] Enable WinRT backend for MSVC2013/2015

Task-number: QTBUG-53925
Change-Id: I674b14df2eee551b21e13e2f5a28de97ed5e4eeb
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>