ethdev: fix missing imissed counter in xstats

ethdev: fix missing imissed counter in xstats

imissed counter has been set as deprecated in commit 49f386542af4
("ethdev: remove driver specific stats") and removed from the
rte_eth_xstats_name_off structure.

The imissed counter has been restored few commits later but has not been
restored in the rte_eth_stats structure. Add it back again.

Fixes: 4eadb8ba11b7 ("ethdev: do not deprecate imissed counter")
Cc: stable@dpdk.org
Signed-off-by: Thibaut Collet <thibaut.collet@6wind.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>