meson.build: Bump libweston major version to 13

Which should have been with commit ed012ee5, 'libweston:
Store view instead of surface, and add flags, to activation data'.

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