forward.h: Add forward declaration of BusObjectImplementation

src/basic/forward.h

@@ -254,6 +254,7 @@ typedef enum UserStorage UserStorage;
 
 typedef struct Bitmap Bitmap;
 typedef struct BPFProgram BPFProgram;
+typedef struct BusObjectImplementation BusObjectImplementation;
 typedef struct CalendarSpec CalendarSpec;
 typedef struct Condition Condition;
 typedef struct ConfigSection ConfigSection;