Fix the build on armhf/aarch64

Summary:
qopengl.h redeclares the functions defined in the epoxy/gl.h and it
results in the build error. this is similar to other .cpp files where
epoxy/gl.h is used.

See: https://build.neon.kde.org/job/xenial_unstable_plasma_kwin_bin_armhf/157

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D10762