CMake: Fix Designer's device skin resources

Qt Widget Designer's device skin resources were added by passing a
directory to qt_internal_add_resources. While rcc takes directories as
input CMake cannot not register correct file dependencies which
manifested as build warnings when using MSBUILD. Also, we had
unnecessary files in the resources (an xcf file, defaultbuttons.conf).

Fix this by listing the files explicitly.

Pick-to: 6.12
Fixes: QTBUG-15164
Change-Id: Ie16d550e4764a0c3b81efa73e38e47d8c5b4f5b7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>