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/condvar.h

@@ -36,6 +36,7 @@
 #include <sys/spl_condvar.h>
 #include <sys/mutex.h>
 #include <sys/time.h>
+#include <sys/errno.h>
 
 /*
  * cv_timedwait() is similar to cv_wait() except that it additionally expects