btrfs-progs: add X inode flag to btrfs(5)

After the discussion in
http://thread.gmane.org/gmane.comp.file-systems.btrfs/36334

the 'X' will be mentioned in the manpage because new e2fsprogs/lsattr
will display it and represents the NOCOMPRESS bit of an inode.

Signed-off-by: David Sterba <dsterba@suse.cz>