Change: Cargo flow legend only shows defined cargo. (#10872)

This removes the large mostly-empty grid of cargo buttons when not using a complex industry set.

(cherry picked from commit 702194cfef)

src/linkgraph/linkgraph_gui.h

@@ -136,6 +136,7 @@ public:
 
 private:
 	LinkGraphOverlay *overlay;
+	size_t num_cargo;
 
 	void UpdateOverlayCompanies();
 	void UpdateOverlayCargoes();