uapi: fix linux/ipv6_route.h userspace compilation errors

Include <linux/in6.h> to fix the following linux/ipv6_route.h userspace
compilation errors:

/usr/include/linux/ipv6_route.h:42:19: error: field 'rtmsg_dst' has incomplete type
  struct in6_addr  rtmsg_dst;
/usr/include/linux/ipv6_route.h:43:19: error: field 'rtmsg_src' has incomplete type
  struct in6_addr  rtmsg_src;
/ust/include/linux/ipv6_route.h:44:19: error: field 'rtmsg_gateway' has incomplete type
  struct in6_addr  rtmsg_gateway;

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>