freebsd: Add fdescfs to the ignored filesystem list

fdescfs is the filesystem mounted on /dev/fd to provide access to file
descriptors via filsystem paths. It is not a real filesystem used on disks.

https://bugzilla.gnome.org/show_bug.cgi?id=748790

sysdeps/freebsd/mountlist.c

@@ -92,6 +92,7 @@ static gboolean ignore_mount_entry(const struct mount_entry *me)
 	static const char ignored[][17] = {
 		"autofs",
 		"devfs",
+		"fdescfs",
 		"fusectl",
 		"linprocfs",
 		"linsysfs",