examples/ipsec-secgw: add per-core packet statistics

examples/ipsec-secgw: add per-core packet statistics

Adding per core packet handling stats to analyze traffic distribution
when multiple cores are engaged.

Since aggregating the packet stats across cores would affect
performance, keeping the feature disabled using compile time flags.

Signed-off-by: Anoob Joseph <anoobj@marvell.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>