fix build on Windows

Change-Id: I023f123a70212b87f7feeeb820dc71a5dd321797
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/coreplugin/iversioncontrol.h

@@ -65,7 +65,7 @@ public:
         OpenMandatory  /*!< Files must always be opened by the VCS */
     };
 
-    class TopicCache
+    class CORE_EXPORT TopicCache
     {
     public:
         virtual ~TopicCache();