[MusicXML export] missing staff in harmony
Existing bug exposed by new test file testHarmony6 in pull request https://github.com/musescore/MuseScore/pull/5958 commit 6ff0246: in a multi-staff context, a harmony element should have a child element staff.
Note that this issue does NOT lead to invalid MusicXML and that the staff is implicitly defined by the following note. It does not seem very likely that this issue would cause import errors.
Trivial to fix.