libepoxy: Fix build break for EGL_CAST dependency
This is a backport [1] to fix the following build error:
| In file included from /home/r60874/upstream/xwayland/tmp/work/armv7at2hf-neon-mx6qdl-fslc-linux-gnueabi/gtk+3/3.22.17-r0/recipe-sysroot/usr/include/epoxy/egl.h:46:0,
| from ../../../gtk+-3.22.17/gdk/wayland/gdkglcontext-wayland.h:32,
| from ../../../gtk+-3.22.17/gdk/wayland/gdkglcontext-wayland.c:24:
| ../../../gtk+-3.22.17/gdk/wayland/gdkglcontext-wayland.c: In function 'gdk_wayland_gl_context_realize':
| ../../../gtk+-3.22.17/gdk/wayland/gdkglcontext-wayland.c:179:43: error: expected expression before 'EGLContext'
| : EGL_NO_CONTEXT,
| ^
[1] https://github.com/anholt/libepoxy/commit/
ebe3a53db1c0bb34e1ca963b95d1f222115f93f8
Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
