wayland-egl: adds CFLAGS for wayland.egl.h include

wayland-egl: adds CFLAGS for wayland.egl.h include

Starting with commit ab0589c6ed ("wayland-egl: remove no longer needed
wayland-client dependency") the wayland-egl.h include was missing leading to a
build failure:

  CC       wayland-egl.lo
wayland-egl.c:33:10: fatal error: wayland-egl.h: No such file or directory
 #include "wayland-egl.h"
          ^~~~~~~~~~~~~~~

Strictly speaking we should be checking for wayland-egl in configure and
propagating its CFLAGS here.

Yet again, the current wayland-egl split is bonkers as the Wayland repo
provides single header, no pkg-config file or library.

That will be resolved at a later stage, but in the meanwhile fix the
build.

Fixes: ab0589c6ed ("wayland-egl: remove no longer needed wayland-client
dependency")
Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
[Emil Velikov: add some text about CFLAGS and current wayland-egl situation]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>