fix too aggressive installation of helper libraries

of course all helper libraries are built statically, so the criterion is
not useful. what is interesting is whether the whole qt configuration is
static, as that determines what will happen with the helper library when
linking the final "actual" artifacts.

Change-Id: I96980c645cb478b2f7a30688b49cb51bec8c9f08
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>