Add core-private dependency to core_api.pro

We want to use private QtCore API in later commits.

Change-Id: If006d9bfd503cdb21b3adf164c0ebf2aa8967bac
Reviewed-by: Michal Klocek <michal.klocek@theqtcompany.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>