libweston: Advertise minor version 2 of zwp_linux_explicit_synchronization_v1

Although we already supported minor version 2 of the explicit sync
protocol, we couldn't advertise it previously, since it was not in any
released version of wayland-protocols. With the release of
wayland-protocols 1.18, which includes minor version 2 of this protocol,
and the recent update in weston to require 1.18, we can now safely
advertise minor version 2.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>