The amd64-gcc12 build was failing with `error: 'devctl_systems' defined
but not used`. Just move it to the C file where it's used.
PR: 271903
Sponsored by: The FreeBSD Foundation
Hooked to devctl_notify, this allows consumers to received events
by subscribing to a system over a generic netlink protocol
Reviewed by: imp, melifaro
Differential Revision: https://reviews.freebsd.org/D37574