Include defs.h before including wxchar.h to make init.h self-contained (able to inclu...

Include defs.h before including wxchar.h to make init.h self-contained (able to include this header in
applications as the only one).
Before with only include wxchar.h it was missing ATTRIBUTE_PRINTF and other things due to wxchar.h not including
defs.h because of defs.h including wxchar.h itself.
We probably don't need to include wxchar.h anymore in init.h then, but I don't want to rely on wxchar.h being
always included from defs.h (if we can rely on that, please change if appropriate)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42109 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775