release: Add ${PKGBASE_REPO} dependency for dvd

A user may build only dvd1.iso, which would fail if the pkgbase-repo
did not already exist. Builds using make release build disc1 before
dvd1 so that case would work, but it must be possible to build only
one artifact.

Fixes: 62d18f8c4c10 ("release: Add -DPKGBASE option to include pkgbase packages")
Sponsored by: The FreeBSD Foundation