Fix private_headers.pri inclusion in tests

Change-Id: Iec8aa54e04b7783b32a2db41ab870cccbadf983e
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

tests/auto/debugger/offsets.pro

 QT -= gui widgets
 include(../qttest.pri)
-include(../../../src/private_headers.pri)
 
 QT += core-private