QTaskTree: Suppress Axivion warnings about captures by reference

Since we pass a loop as a context object to the connection it's
guaranteed that the lambda is executed when the context object
still exists, thus the doneWith must also exist as it's
destructed after the loop.

Pick-to: 6.11
Task-number: QTBUG-146069
Axivion-Id: qt_Add-ons_qttasktree_dev_linux:SV7
Axivion-Id: qt_Add-ons_qttasktree_dev_linux:SV8
Change-Id: I03dc4cbf3dabd4a02627288b34b875a00c7a4927
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>