egl/wayland: Always use in-tree wayland-egl-backend.h

egl/wayland: Always use in-tree wayland-egl-backend.h

A recent patchset to Wayland[0] migrated Mesa's libwayland-egl backend
into Wayland itself, so implementations could provide backends. Mesa
still uses its own, and the two have already diverged[1].

The include from egl_dri2.h could pick up either the installed Wayland
wayland-egl-backend.h (with a 'driver_private' member), or the Mesa
internal wayland-egl-backend.h (with a 'private' member), failing the
build in the first instance.

Add an explicit directory prefix to the include, so we always get our
in-tree version.

[0]: https://patchwork.freedesktop.org/series/31663/
[1]: https://cgit.freedesktop.org/wayland/wayland/commit/?id=9fa60983b579

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105103
Fixes: 198af27c679c ("wayland-egl: rename wayland-egl-{priv,backend}.h")