freebsd: changes necessary to coexist with dtrace in tree

Fix header conflicts when building zfs with openzfs as a vendor import.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10497

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

@@ -29,7 +29,9 @@
 #include <sys/uio.h>
 #include <sys/mutex.h>
 #include <sys/u8_textprep.h>
+#ifdef BUILDING_ZFS
 #include <sys/vnode.h>
+#endif
 
 typedef int ddi_devid_t;