move framework bundle rule to DarwinGCC.qbs

It's technically possible to create frameworks on iOS.

Change-Id: I68bd37c46e4d6fe76aae16a1fab4879ef3f3f741
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>