FreeBSD: make adjustments for the standalone environment

In FreeBSD, there are three compile environments that are supported:
user land, the kernel and the bootloader / standalone. Adjust the
headers to compile in the standalone environment. Limit kernel-only
items from view when _STANDALONE is defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Warner Losh <imp@FreeBSD.org>
Closes #10998

include/os/freebsd/spl/sys/kmem_cache.h

@@ -30,6 +30,7 @@
 #ifndef _SPL_KMEM_CACHE_H
 #define	_SPL_KMEM_CACHE_H
 
+#ifdef _KERNEL
 #include <sys/taskq.h>
 
 /* kmem move callback return values */
@@ -46,4 +47,6 @@ extern void spl_kmem_cache_set_move(kmem_cache_t *,
 
 #define	kmem_cache_set_move(skc, move)	spl_kmem_cache_set_move(skc, move)
 
+#endif /* _KERNEL */
+
 #endif