examples/ipsec-secgw: fix ESP flow error log

examples/ipsec-secgw: fix ESP flow error log

Function create_ipsec_esp_flow returns a negative number in case of any
failure and we are passing this to strerror to display the error message.
But strerror()'s argument cannot be negative.
In case of failure, displaying exact error message to console is handled
in create_ipsec_esp_flow function.
So it is not required to print the error message again using strerror.
This patch will remove the unnecessary calling of strerror function
to fix the negative argument passing to strerror issue.

Coverity issue: 357691
Fixes: 6738c0a95695 ("examples/ipsec-secgw: support flow director")

Signed-off-by: Praveen Shetty <praveen.shetty@intel.com>
Acked-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Acked-by: Anoob Joseph <anoobj@marvell.com>