meson: Fix to make QAPI generator output depend on main.py

@qapi_gen_depends is missing scripts/qapi/main.py.  Fix that, and drop
a duplicate scripts/qapi/common.py.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230428105429.1687850-5-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>