MSWordExportBase: fix export of header/footer in case of multiple columns

Regression from 263938c4a8789d881f8e736d317b6bcc09c3bce5 (fdo#73596
[DOCX] Multiple Columns in Index, 2014-02-13), header / footer was lost
in multi-column section. This fixes both DOC and DOCX export.

Change-Id: Icb30ec882b38853e25289299d8f829a5cf56e6d8