Force linking of all wxMSW wxMediaCtrl backends in mediactrl sample.

Force linking of all wxMSW wxMediaCtrl backends in mediactrl sample.

Force the linker to include all the backends in the sample executable instead
of discarding them because they are not used directly to allow testing all of
them in the sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65485 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775