Fix missing prefix

Change-Id: I0a9df00f90b63059e6446278d7a98d1cd902de4b
DOCX filter: improve qFormat export handling

The old rule was: write qFormat for everything that's not a custom
style.

The new rule: write qFormat for everything that's a custom style + have
a whitelist of non-custom, but qFormat styles.

This matches better what Word does (whitelist is from the latent style
section of an empty document, created by Word).

Change-Id: Ie7a0802e886c41b8d26ca9aa154913aa2f3ff87a