doc: fix the compile error of txtimestamp.c

Vinson reported:

  HOSTCC  Documentation/networking/timestamping/txtimestamp
Documentation/networking/timestamping/txtimestamp.c:64:8: error:
redefinition of ‘struct in6_pktinfo’
 struct in6_pktinfo {
        ^
In file included from /usr/include/arpa/inet.h:23:0,
                 from Documentation/networking/timestamping/txtimestamp.c:33:
/usr/include/netinet/in.h:456:8: note: originally defined here
 struct in6_pktinfo
        ^

After we sync with libc header, we don't need this ugly hack any more.

Reported-by: Vinson Lee <vlee@twopensource.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>