meson.build: Bump libweston major version

With commit 'compositor: Remove desktop zoom' weston_output_zoom was removed
from weston_output thus needing a libweston major bump.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>