libsysdecode: fix nlm_flag regex in mktables

Some NLM_F_ definitions contain multiple underscores in their name; this
should pick them up.

Reviewed by: kp, Ishan Agrawal <iagrawal9990@gmail.com>
Fixes: 4c932a4d45fb ("netlink: decode netlink message flags symbolically")
Sponsored by: The FreeBSD Foundation
Pull Request: https://github.com/freebsd/freebsd-src/pull/2340