MAINTAINERS: add btrfs IRC link

We haven't had an IRC link before but now it's a good time to announce
where to reach the community.

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

MAINTAINERS

@@ -3803,6 +3803,7 @@ L:	linux-btrfs@vger.kernel.org
 S:	Maintained
 W:	http://btrfs.wiki.kernel.org/
 Q:	http://patchwork.kernel.org/project/linux-btrfs/list/
+C:	irc://irc.libera.chat/btrfs
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git
 F:	Documentation/filesystems/btrfs.rst
 F:	fs/btrfs/