appletalk: Add atalk.h header files to MAINTAINERS file

Add the path names here so that git-send-email can pick up the
netdev@vger.kernel.org Cc line automatically for a patch that
only touches the headers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>