Make access to xkbcommon_workaround.h more generic

It makes more sense to keep this workaround header together with the other
libxkbcommon files for a better access point since it's used by several *.pro
files.

Change-Id: I63d4eb58f6e7f3852834e41c4b6e058a2c962233
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>