Only use packing in horizontal direction for toolbar controls

Tool packing seems to be relevant only in the major toolbar direction,
i.e. horizontally when the controls are shown, and adding it in vertical
direction too made the toolbar too tall.

src/msw/toolbar.cpp

@@ -540,7 +540,7 @@ wxSize wxToolBar::MSWGetFittingtSizeForControl(wxToolBarTool* tool) const
     }
 
     // Also account for the tool padding value.
-    size += wxSize(m_toolPacking, m_toolPacking);
+    size.x += m_toolPacking;
 
     return size;
 }