Make sure there's a single global RTTI for SfxLinkItem

...as otherwise dynamic_cast<SfxLinkItem*>(...) could fail at least on macOS (as
observed with CppunitTest_sc_screenshots)

Change-Id: I4a41a8b085b839598119791637f79cd467ad4130

include/sfx2/app.hxx

@@ -97,7 +97,7 @@ enum class SfxToolsModule
     LAST = Basic
 };
 
-class SfxLinkItem : public SfxPoolItem
+class SFX2_DLLPUBLIC SfxLinkItem : public SfxPoolItem
 {
     Link<SfxPoolItem*, void> aLink;
 public: