Drop XFixes include in cursor.cpp

Summary:
We no longer need to include xcb/xfixes.h in cursor.cpp because
X11Cursor moved to its own file.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18133