build: use project_source_root()

Fixes Meson warning:

../../git/weston/doc/sphinx/meson.build:40: WARNING: Project targets '>=
0.63.0' but uses feature deprecated since '0.56.0': meson.source_root.
use meson.project_source_root() or meson.global_source_root() instead.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>