cifs: add cifs_common directory to MAINTAINERS file

With some files moving into the cifs_common directory, we need
to add it to the CIFS entry in the MAINTAINERS file.
Suggested-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

MAINTAINERS

@@ -4629,6 +4629,7 @@ W:	http://linux-cifs.samba.org/
 T:	git git://git.samba.org/sfrench/cifs-2.6.git
 F:	Documentation/admin-guide/cifs/
 F:	fs/cifs/
+F:	fs/cifs_common/
 
 COMPACTPCI HOTPLUG CORE
 M:	Scott Murray <scott@spiteful.org>