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

sys/sys/alq.h

@@ -37,6 +37,7 @@
 #ifndef _SYS_ALQ_H_
 #define	_SYS_ALQ_H_
 
+#ifdef _KERNEL
 /*
  * Opaque type for the Async. Logging Queue
  */
@@ -143,4 +144,5 @@ alq_post(struct alq *alq, struct ale *ale)
 	alq_post_flags(alq, ale, 0);
 }
 
+#endif	/* _KERNEL */
 #endif	/* _SYS_ALQ_H_ */