libepoxy: reinstate mesa egl cflags patch

libepoxy: reinstate mesa egl cflags patch

This is still required when building libepoxy against mesa egl without
x11 (GL).

Mesa needs a preprocessor define to avoid including X11 headers and
since this is not forwarded into epoxy.pc for other
libraries/applicatons that link against it it will fail.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>