iOS: Don't pass an specific version to -sdk

Let's just use 'iphone' and 'iphonesimulator', otherwise they
get hardcoded in the installers (mkspecs/qdevice.pri).

Change-Id: Icd151b2b38ff1060394bcde07fe03ba847e84279
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>