d/postinst: Set proper ownership on the DB file (Closes: #1081562)

The latest version of miniflux can't connect to the database after a fresh installation. This is due to incorrect ownership of /etc/miniflux/database file which is owned by root (and correctly having the permissions 0600). After changes in bug #1078416 🎉, miniflux no longer runs as root user and instead runs as miniflux user. This user can't read the database file. The daemon silently falls back to using built in defaults and fails to connect to PostgreSQL database. Fix this by changing ownership of the file.

Signed-off-by: Sunil Mohan Adapa sunil@medhas.org