iOS: Add explicit dependency on the Foundation framework for QtCore

First of all it needed on both OSX and iOS, and secondly it's required
for our Qt APIs, now that we provide conversion functions to Foundation
types such as NSURL and NSString, so it should be part of LIBS, not
LIBS_PRIVATE.

Change-Id: Ida4977d6aa0e76b3d1b8b565672c00a8526e3381
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>