...as it is included from other include/vcl/*.hxx files.  (And moving it there
helps the heuristic in an upcoming improvement of loplugin:dyncastvisibility.)

Change-Id: I15e061fbaba8e82235afb5b312f7b38f26eec900
Reviewed-on: https://gerrit.libreoffice.org/50123Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>