test/pipeline: fix return type of stub miss

test/pipeline: fix return type of stub miss

New warning with gcc 8.1:
The table_action_stub_miss() function was incorrectly
declared as returning a typedef of
rte_pipeline_table_action_handler_miss, instead of what it
should have been, an int.
Changed this to declare the function as returning an int, and
now when the action_handler_miss is set to table_action_stub_miss
(cast as rte_pipeline_table_action_handler_miss), gcc 8.1 no
longer complains about the cast.

Fixes: 5205954791cb ("app/test: packet framework unit tests")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>
Reviewed-by: Reshma Pattan <reshma.pattan@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>