Missed adding netgraph to mtree in r368443:

New Netgraph module ng_macfilter:

Macfilter to route packets through different hooks based on sender MAC address.

Based on ng_macfilter written by Pekka Nikander

Sponsered by Retina b.v.

Reviewed by: afedorov
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D27268