Compilation fix for --disable-precomp-headers in combination with --disable-all-features .

Fix an error for compile step of "Linux i386 wxGTK trunk no features" buildbot by forward declaring wxWindow class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76550 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775