Remove epoxy/egl.h from kwinglutils_func.h

Summary:
It's not needed inside libkwinglutils. The only required usage is from
abstract_egl_backend.h where the include is moved to.

This removes an fixx11h.h from a kwin lib.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3447