selftests: mptcp: remove duplicate include in mptcp_inq.c

'sys/ioctl.h' included in 'mptcp_inq.c' is duplicated.

Reported-by: ZealRobot <zealci@zte.com.cn>
Signed-off-by: Ye Guojin <ye.guojin@zte.com.cn>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20211210071424.425773-1-ye.guojin@zte.com.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>