Add missing header for minimalistic builds not using PCH.

Add missing header for minimalistic builds not using PCH.

wxWindow might not be fully declared in dcbase.cpp but we need its full
declaration for wxDCImpl::InheritAttributes() so explicitly include
wx/window.h -- while this is usually already included from somewhere else it
might not be in minimal builds with a lot of features disabled.

Closes #13380.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68522 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775