[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>

include/linux/Kbuild

@@ -239,6 +239,7 @@ unifdef-y += ipc.h
 unifdef-y += ipmi.h
 unifdef-y += ipv6.h
 unifdef-y += ipv6_route.h
+unifdef-y += ip6_tunnel.h
 unifdef-y += isdn.h
 unifdef-y += isdnif.h
 unifdef-y += isdn_divertif.h