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: NVinson Lee <vlee@twopensource.com>
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/timestamping/txtimestamp.c

@@ -59,14 +59,6 @@
 #include <time.h>
 #include <unistd.h>
 
-/* ugly hack to work around netinet/in.h and linux/ipv6.h conflicts */
-#ifndef in6_pktinfo
-struct in6_pktinfo {
-	struct in6_addr	ipi6_addr;
-	int		ipi6_ifindex;
-};
-#endif
-
 /* command line parameters */
 static int cfg_proto = SOCK_STREAM;
 static int cfg_ipproto = IPPROTO_TCP;