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/mount.h

@@ -31,8 +31,9 @@
 
 #include <sys/param.h>
 #include_next <sys/mount.h>
+#ifdef BUILDING_ZFS
 #include <sys/vfs.h>
-
+#endif
 #define	MS_FORCE	MNT_FORCE
 #define	MS_REMOUNT	MNT_UPDATE