QmakeProject: Fix mergeLists which lead to cumulative parse being ignored

Any .pri files only found via cumulative parsing were ignored.

Task-number: QTCREATORBUG-13181
Change-Id: I1a7a639097b38a4ce1de01824be196a0c2781e84
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>