Summary:
With recent Mesa updates (19.2.1), MESA_EGL_NO_X11_HEADERS doesn't have
any effect. So, let's include fixx11h.h instead.

Test Plan: I am able to build kwin once again.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24840