Include wx/defs.h before testing for wxUSE_CHOICEDLG.

Fix compilation of this header if it's the first one to be included from user
code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75011 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775