Move DSS_HIDEPREFIX fallback definition to the central header.

Do it only once now that it's used in 3 different places (it recently started
to be used in src/msw/anybutton.cpp as well) instead of defining it thrice.

This also fixes MinGW build.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75930 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775