don't generate invalid xml files

VmlCommentExporter::StartShape already adds a XML_type to the attribute
list, so we would be exporting the same attribute twice which makes the
xml file invalid. This file could therefore not be imported again.

Change-Id: I2aed2805ea59c15b1f7e60ab9cab10f7a8e81412
Reviewed-on: https://gerrit.libreoffice.org/6980Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

oox/source/export/vmlexport.cxx

@@ -1022,7 +1022,7 @@ sal_Int32 VMLExport::StartShape()
     // add style
     m_pShapeAttrList->add( XML_style, m_pShapeStyle->makeStringAndClear() );
 
-    if ( nShapeElement >= 0 )
+    if ( nShapeElement >= 0 && !m_pShapeAttrList->hasAttribute( XML_type ) )
     {
         if ( bReferToShapeType )
         {