[IPV6]: Add ip6_tunnel.h to headers_install

The Mobile IPv6 package (http://www.mobile-ipv6.org/software/) needs
this header file to build the tunnelctl component. The header
already looks sanitized so is safe to export.

Signed-off-by: Deepak Saxena <dsaxena@mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>