Fix typehint for getServiceLocator().

It's always `ServiceLocatorInterface` because getServiceLocator() returns the contents of $this->serviceLocator which is set by setServiceLocator(ServiceLocatorInterface $sl).
Merge pull request #5471 from Thinkscape/patch-1

Fix typehint for getServiceLocator().
Merge branch 'hotfix/5471'

Close #5471
Merge remote-tracking branch 'zf2/master' into prod