de-macroize XINTERFACE_COMMON_IMPL

Change-Id: I7cffc8a111dd939272c6dbef8827b6f5c04a4590
Reviewed-on: https://gerrit.libreoffice.org/8455
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>