pbrd: add frr-vrf to the list of implemented yang modules

pbrd: add frr-vrf to the list of implemented yang modules

PR #6376 introduced a VRF leafref in the frr-interface YANG module.
That change exposed a bug in the northbound layer that is causing
pbrd to crash under certain circumstances. Even though pbrd wasn't
converted to the new northbound model yet, make it implement the
frr-vrf module in order to work around this problem. This is a
temporary fix until a better solution is available.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>