selftests/net: in rxtimestamp getopt_long needs terminating null entry

getopt_long requires the last element to be filled with zeros.
Otherwise, passing an unrecognized option can cause a segfault.

Fixes: 16e78122 ("selftests/net: Add a test to validate behavior of rx timestamps")
Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/rxtimestamp.c

@@ -115,6 +115,7 @@ static struct option long_options[] = {
 	{ "tcp", no_argument, 0, 't' },
 	{ "udp", no_argument, 0, 'u' },
 	{ "ip", no_argument, 0, 'i' },
+	{ NULL, 0, NULL, 0 },
 };
 
 static int next_port = 19999;