Need to add destructor to WBoxLayout

Impl::Grid is not exported, so the caller can't instantiate
~WBoxLayout(). It needs to be exported.

Author: RockinRoel

src/Wt/WBoxLayout.C

@@ -38,6 +38,9 @@ WBoxLayout::WBoxLayout(LayoutDirection dir)
   : direction_(dir)
 { }
 
+WBoxLayout::~WBoxLayout()
+{ }
+
 void WBoxLayout::addItem(std::unique_ptr<WLayoutItem> item)
 {
   insertItem(count(), std::move(item), 0, None);

src/Wt/WBoxLayout.h

@@ -107,6 +107,8 @@ class WT_API WBoxLayout : public WLayout
    */
   WBoxLayout(LayoutDirection dir);
 
+  virtual ~WBoxLayout() override;
+
   virtual void addItem(std::unique_ptr<WLayoutItem> item) override;
   virtual std::unique_ptr<WLayoutItem> removeItem(WLayoutItem *item) override;
   virtual WLayoutItem *itemAt(int index) const override;