ofw_graph(9): hook it up to the Makefile

Otherwise, the man page is not installed. Add appropriate MLINKS.

Reviewed by: manu
MFC after: 3 days
Fixes: 9a4eed0be20c ("ofw_graph: Add functions for graph bindings")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36932