use approximate_char_width instead of GetTextWidth('X')

Change-Id: I4c25b07c1c5d80b70b801fd61b711a00d5194c04

vcl/source/control/combobox.cxx

@@ -1131,7 +1131,7 @@ Size ComboBox::CalcSize( sal_uInt16 nColumns, sal_uInt16 nLines ) const
 
     // width
     if ( nColumns )
-        aSz.Width() = nColumns * GetTextWidth(rtl::OUString(static_cast<sal_Unicode>('X')));
+        aSz.Width() = nColumns * approximate_char_width();
     else
         aSz.Width() = aMinSz.Width();