exclude qbs subprojects from lupdate message collection

the qbs translations should come from the qbs lib itself.

requires qt 5.1.2 to have an effect.

Change-Id: I0aabeaf24d06c9070c2de6e5a0bb9a8ddd6c06c1
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>