eventdev: fix build with icc

eventdev: fix build with icc

ICC complains about variable being used before its value is set.
Since the variable is only assigned in the for loop,
its declaration is moved inside and is initialized.

lib/librte_eventdev/rte_event_timer_adapter.c(708): error #592:
variable "ret" is used before its value is set
        RTE_SET_USED(ret);

Fixes: 6750b21bd6af ("eventdev: add default software timer adapter")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>