Allow GParted to read the UUID from older btrfs-show command

All the code was there in btrfs::read_uuid() to read the UUID from the
btrfs-show command.  It just needed enabling.

This is only relevant when: (1) the blkid command is unavailable as
GParted primarily reads the UUID through the FS_Info cache using the
blkid command, and (2) the btrfs command is unavailable and the btrfs
module uses the older btrfs-show command instead.

src/btrfs.cc

@@ -78,6 +78,7 @@ FS btrfs::get_filesystem_support()
 		{
 			fs .read = GParted::FS::EXTERNAL ;
 			fs .read_label = FS::EXTERNAL ;
+			fs .read_uuid = FS::EXTERNAL ;
 		}
 
 		//Resizing of btrfs requires btrfsctl, mount, umount