[balooctl] Flush buffered "Indexing <file>" line when indexing starts

Summary:
Without the flush, the line is buffered until the endl from the finished
flushes the output, i.e. the output is only visible after completion of
a file, not while it is processed.

See also D15994

Test Plan: same as for D15994, but now visible in balooctl monitor instead of dbus-monitor

Reviewers: Baloo, poboiko, ngraham, Frameworks

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D15995