[Extractor] Fix progress reporting

Summary:
Commit 125b4db3230b ("Port endl to \n or std::cout/cerr as in qt5.15 endl
is Qt:: namespaced") removed the implicit "endl" flush. As a result,
the startedIndexing and finishedIndexing signals would be issued at the
same time.

This is especially bad when indexing large files, as both the KInfoCenter
qml component and balooctl monitor would report the previous file (or
no file at all, if only a single file is changed).

Test Plan:
start kinfocenter / balooctl monitor
touch some huge PDF files

Reviewers: Baloo, ngraham, mlaurent, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28953