sys/alq.h: Kernel only file, mark as such

The alq interfaces are 100% in-kernel, so make this whole file #ifdef
_KERNEL. There's no users of this in the tree outside of the kernel, nor
does it define anything that could be useful at peeking into the state
of alq.

Sponsored by: Netflix

(cherry picked from commit 242d6201a6c840a68dc19b0df1157568751e1391)