Don't use MESA_EGL_NO_X11_HEADERS
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