Fix crashes on Mac

Seems Mac and/or clang need these to be exported

BUGS: 345765

interfaces/viewerinterface.h

@@ -23,7 +23,7 @@ namespace Okular {
  * This interface can be used to control some more or less advanced features of a document
  * viewer.
  */
-class ViewerInterface
+class OKULAR_EXPORT ViewerInterface
 {
     public:
         virtual ~ViewerInterface() {}

kdocumentviewer.h

@@ -10,6 +10,8 @@
 #ifndef _KDOCUMENTVIEWER_H_
 #define _KDOCUMENTVIEWER_H_
 
+#include "okular_part_export.h"
+
 #include <QtCore/QStringList>
 
 class KUrl;
@@ -19,7 +21,7 @@ class KUrl;
  *
  * This interface describe briefly the basic functions of a document viewer.
  */
-class KDocumentViewer
+class OKULAR_PART_EXPORT KDocumentViewer
 {
     public:
         virtual ~KDocumentViewer() {}