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/taskq.h

@@ -26,6 +26,8 @@
 #ifndef	_SYS_TASKQ_H
 #define	_SYS_TASKQ_H
 
+#ifdef _KERNEL
+
 #include <sys/types.h>
 #include <sys/proc.h>
 #include <sys/taskqueue.h>
@@ -112,4 +114,11 @@ void	taskq_resume(taskq_t *);
 }
 #endif
 
+#endif /* _KERNEL */
+
+#ifdef _STANDALONE
+typedef int taskq_ent_t;
+#define	taskq_init_ent(x)
+#endif /* _STANDALONE */
+
 #endif	/* _SYS_TASKQ_H */