clang-format: place sys/systm.h immediately after sys/param.h

[arichardson]

Summary

Should fix one of the objections to r366993

[emaste]

LGTM. I found a few examples in sys/ that don't follow this but most do, and I discussed w/ kib in email that we should describe this in style(9) too.

head/.clang-format

#ifdef INET6
         case INC_ISIPV6:
                 SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
                 SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
                 break;
 #endif
         }
         SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
         SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
+        SipHash_Update(&ctx, &irs, sizeof(irs));
         SipHash_Update(&ctx, &flags, sizeof(flags));
         SipHash_Update(&ctx, &secmod, sizeof(secmod));
         SipHash_Final((u_int8_t *)&siphash, &ctx);
 
         return (siphash[0] ^ siphash[1]);
 }
 
 static tcp_seq