examples/ipsec-secgw: move global array from header

examples/ipsec-secgw: move global array from header

When STATS_INTERVAL is set to a non-zero value the
core_statistics array will be defined in multiple
compilation units and this can trigger a linker error
on particular environments. In order to fix this the
core_statistics definition was moved out of the header file.

Fixes: 1329602b6c8f ("examples/ipsec-secgw: add per-core packet statistics")
Cc: stable@dpdk.org
Signed-off-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>