userspace: mark arguments used

Reviewed-by: Alejandro Colomar <alx.manpages@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13110

cmd/zfs/zfs_main.c

@@ -751,6 +751,8 @@ note_dev_error(int err, int fd)
 		    "(As a workaround, "
 		    "try \"zfs send [...] | cat > /dev/null\")\n"));
 	}
+#else
+	(void) err, (void) fd;
 #endif
 }