examples/ipsec-gw: fix gcc 10 maybe-uninitialized warning

examples/ipsec-gw: fix gcc 10 maybe-uninitialized warning

gcc 10.0.1 reports:

../examples/ipsec-secgw/ipsec_process.c: In function ‘ipsec_process’:
../examples/ipsec-secgw/ipsec_process.c:132:34:
error: ‘grp.m’ may be used uninitialized in this function
 [-Werror=maybe-uninitialized]
  132 |    grp[n].cnt = pkts + i - grp[n].m;
      |                            ~~~~~~^~

This is a correct warning for the initial execution of the statement.
However, it is the design of the loop that grp[0].cnt will later be
written with the correct value using an initialized grp[0].m before it
is used.

In order to remove the warning, initialize grp[0].m for the initial and
unused calculation of grp[0].cnt.

Fixes: 3e5f4625dc17 ("examples/ipsec-secgw: make data-path to use IPsec library")
Cc: stable@dpdk.org
Suggested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>