A fix for VC6 compilation: define missing priority constants.

{ABOVE,BELOW}_NORMAL_PRIORITY_CLASS aren ot defined in VC6 headers, see
http://buildbot.tt-solutions.com/wx/builders/XPSP2%20VC6%20wxMSW%20trunk%20release/builds/2943/steps/compile/logs/stdio
so define them ourselves.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73415 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775