crypto/scheduler: fix build with old gcc

crypto/scheduler: fix build with old gcc

Seen with gcc 4.9.2:
drivers/crypto/scheduler/scheduler_multicore.c:286:2: error:
'for' loop initial declarations are only allowed in C99 or C11 mode
  for (uint16_t i = 0; i < sched_ctx->nb_wc; i++)
  ^

Fixes: 4c07e0552f0a ("crypto/scheduler: add multicore scheduling mode")

Signed-off-by: Jan Blunck <jblunck@infradead.org>