Fix f66baa44: for-loop is no longer increasing "i"

During conversion it was overlooked that the for-loop used to do
this. Oops.

src/ai/ai_gui.cpp

@@ -92,6 +92,8 @@ struct AIListWindow : public Window {
 					this->selected = i;
 					break;
 				}
+
+				i++;
 			}
 		}
 	}