Include xkb.h

Summary:
I don't see why the header cannot be included. I guess this is some old
legacy, this change synchronizes it with Qt, which also includes xkb.

Include the header in only one place and remove struct definitions which are defined
upstream.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23043