eglmesaext: add forward declaration for struct wl_buffers

eglmesaext: add forward declaration for struct wl_buffers

The user does not need to know the specifics of the struct, as only a
pointer to it is used.

Just forward declare the struct making the header self-contained.

v2: Remove deprecation warning text/bugzilla - patch does no help there.

Cc: Greg V <greg@unrelenting.technology>
Fixes: 5cddb1ce3c9 ("wayland: Add an extension to create wl_buffers from
EGLImages")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (v1)