Fix static builds with ICU

Make sure we actually link against the static version of the ICU libs for
static builds.

Task-number: QTBUG-29478

Change-Id: Ida7b439f11c5393bee43bfe804f9ec84bf272b34
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>