iOS: Fix linking against QtCore

After introducing support for QSysInfo::macVersion() for iOS we need
clients to link to UIKit when using QtCore, as that's where the UIDevice
class lives.

Change-Id: I0a9c2e2506c61ac5619fd000ebd10c2ab9e037cf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/kernel.pri

@@ -98,6 +98,9 @@ mac:!nacl {
                 kernel/qcore_mac.cpp
        OBJECTIVE_SOURCES += \
                 kernel/qcore_mac_objc.mm
+
+       # We need UIKit for UIDevice
+       ios: LIBS_PRIVATE += -framework UIKit
 }
 
 nacl {