Disable qtdeclarative on iOS

It's also currently disabled in qtbase' configure, but the tweak there will
go away soon as the wip/v4 branch of qtdeclarative is subject to _enabling_
on iOS.

Change-Id: I8c17387ada467235d79d40a6cd6f5d0f8c173186
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>