add headerfile for tcp_v6_check use

[axkum10]

Hello John,

I added a header file to solve below compilation error.

  CC [M]  /home/dc-user/homa/HomaModule/homa_outgoing.o
In file included from /home/dc-user/homa/HomaModule/homa_outgoing.c:13:
/home/dc-user/homa/HomaModule/homa_hijack.h: In function ‘homa_hijack_set_hdr’:
/home/dc-user/homa/HomaModule/homa_hijack.h:68:18: error: implicit declaration of function ‘tcp_v6_check’; did you mean ‘tcp_v4_check’? [-Werror=implicit-function-declaration]
   h->checksum = ~tcp_v6_check(skb->len, &peer->flow.u.ip6.saddr,
                  ^~~~~~~~~~~~
                  tcp_v4_check
cc1: some warnings being treated as errors

Thanks

[johnousterhout]

Thanks for pointing this out. I'm going to fix it a slightly different way (there is a different function to use in older versions of Linux), which will match what is in the linux_4.18.0 branch.