Make sure there's a single global RTTI for pdfio::PDFElement and derived

...as otherwise dynamic_cast<xmlsecurity::pdfio::PDFNameElement*>(...) in
xmlsecurity/qa/unit/pdfsigning/pdfsigning.cxx will fail at least on macOS,
causing CppunitTest_xmlsecurity_pdfsigning to fail.

Change-Id: I7c41c994a1e6145b4740a97ffe47d0c42c4e3ca0

xmlsecurity/inc/pdfio/pdfdocument.hxx

@@ -35,7 +35,7 @@ class PDFArrayElement;
 class PDFStreamElement;
 
 /// A byte range in a PDF file.
-class PDFElement
+class XMLSECURITY_DLLPUBLIC PDFElement
 {
 public:
     virtual bool Read(SvStream& rStream) = 0;