QNX: Fix logging of QtDebugMsg in slog2 backend

With the introduction of QtInfoMsg in commit ef6279fd

 we also changed
the mapping of Qt to slog2 levels: QtInfoMsg now ends up as SLOG2_DEBUG1,
instead of SLOG2_INFO. Anyhow, we didn't change the default buffer
verbosity level accordingly.

Task-number: QTBUG-51378
Change-Id: Ia464f9e5a31e19413902e877d4f2be0ba6d340db
Reviewed-by: Dan Cape <dcape@qnx.com>
Reviewed-by: Janne Koskinen <janne.p.koskinen@theqtcompany.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>