MAINTAINERS: add btrfs file entries for include directories

Add file entries for btrfs header files.
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

MAINTAINERS

@@ -2904,6 +2904,8 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs.git
 S:	Maintained
 F:	Documentation/filesystems/btrfs.txt
 F:	fs/btrfs/
+F:	include/linux/btrfs*
+F:	include/uapi/linux/btrfs*
 
 BTTV VIDEO4LINUX DRIVER
 M:	Mauro Carvalho Chehab <mchehab@s-opensource.com>