netlink: move NETLINK define to opt_global.h

Move the NETLINK define into opt_global.h so we can rely on it being
set correctly, without having to remember to include opt_netlink.h.
This ensures that the NETLINK define is correctly set. If not we
may end up with unloadable modules, due to missing symbols (such as
nlmsg_get_group_writer).

PR: 274306
Reviewed by: imp, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42179