Remove _KERNEL guard from if_npflog.h?

[dhgutteridge]

Presently, if_npflog.h has a _KERNEL guard on it:
https://github.com/rmind/npf/blob/2efbe2828bc507896248598ffb1026653cbfed2c/src/kern/if_npflog.h#L28C1-L30C7
In discussion with other NetBSD developers, we weren't sure why that was, and there was a suggestion to simply remove it. (The equivalent if_pflog.h does not have this restriction. Part of the context being, evenutally we will be removing PF from NetBSD.) I thought I'd ask about that here.