Non-implicit virtual SvxPageItem dtor

...otherwise, as the class is SVX_DLLPUBLIC and---for better or worse---
SVX_DLLPUBLIC is shared between Library_svxcore and Library_svx, all .cxx in
Library_svxcore that happen to include svx/pageitem.hxx would emit it (as well
as those in Library_svx, of course).  But at least clang-cl at /O0 would want
to put SvxPageItem's (base-class) vtable in place in the dtor, so would also
emit the vtable, so would depend on SvxPageItem::operator==, but that is defined
in pageitem.cxx in Library_svx, against which Library_svxcore doesn't link.

Change-Id: Ibfc393c1de3e0bd6621c057d88b1b0f272820154

include/svx/pageitem.hxx

@@ -72,6 +72,8 @@ public:
     SvxPageItem( const sal_uInt16 nId );
     SvxPageItem( const SvxPageItem& rItem );
 
+    virtual ~SvxPageItem();
+
     virtual SfxPoolItem*     Clone( SfxItemPool *pPool = nullptr ) const override;
     virtual bool             operator==( const SfxPoolItem& ) const override;
 

svx/source/items/pageitem.cxx

@@ -51,6 +51,8 @@ SvxPageItem::SvxPageItem( const SvxPageItem& rItem )
     eUse        = rItem.eUse;
 }
 
+SvxPageItem::~SvxPageItem() {}
+
 // Clone
 SfxPoolItem* SvxPageItem::Clone( SfxItemPool * ) const
 {