cifs: missing directory in MAINTAINERS file

The include/uapi/linux/cifs directory (not just fs/cifs and
fs/smbfs_common) should be included in cifs entry in the
MAINTAINERS file.

Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>