selftests/net: so_txtime: remove unneeded semicolon

Eliminate the following coccicheck warning:
./tools/testing/selftests/net/so_txtime.c:199:3-4: Unneeded semicolon

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>