drop_monitor: Add missing uAPI file to MAINTAINERS file

Fixes: 6e43650c

 ("add maintainer for network drop monitor kernel service")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>