Fix compile error

Change-Id: Iaf68a88d8563a0086766203b64d73681abd1827b
Reviewed-by: Christian Stenger <christian.stenger@digia.com>

src/plugins/projectexplorer/projectnodes.h

@@ -338,6 +338,7 @@ private:
 
     // let project & session emit signals
     friend class ProjectNode;
+    friend class FolderNode;
     friend class SessionNode;
     friend class Node;
 };