tools: tcptracer: fix alignement in tcp_ipv6_event_t

On IPv6, tcptracer ports always appears as zeros:

Tracing TCP established connections. Ctrl-C to end.
T  PID    COMM             IP SADDR            DADDR            SPORT  DPORT
X  7055   nc               4  127.0.0.1        127.0.0.1        49476  9999
C  7074   nc               4  127.0.0.1        127.0.0.1        49478  9999
X  7074   nc               4  127.0.0.1        127.0.0.1        49478  9999
C  7085   nc               6  [::]             [0:0:0:1::]      0      0
X  7085   nc               6  [::]             [0:0:0:1::]      0      0
C  7086   nc               6  [::]             [0:0:0:1::]      0      0

This seems related to alignment issue wrt to the __int128 type in
tcp_ipv6_event_t structure. Moving the u8 field ip to the end of the
structure fixes the issue.

Fixes iovisor#2781

tools/tcptracer.py

@@ -73,12 +73,12 @@
  u32 type;
  u32 pid;
  char comm[TASK_COMM_LEN];
- u8 ip;
  unsigned __int128 saddr;
  unsigned __int128 daddr;
  u16 sport;
  u16 dport;
  u32 netns;
+ u8 ip;
 };
 BPF_PERF_OUTPUT(tcp_ipv6_event);