some prototype fixes in RPL

sys/net/rpl/of_mrhof.h

@@ -32,4 +32,4 @@
  */
 #define ETX_RANK_MULTIPLIER (0x80)
 
-rpl_of_t *rpl_get_of_mrhof();
+rpl_of_t *rpl_get_of_mrhof(void);

sys/net/rpl/rpl_structs.h

@@ -270,7 +270,7 @@ typedef struct rpl_of_t {
     void (*reset)(rpl_dodag_t *);
     void (*parent_state_callback)(rpl_parent_t *, int, int);
     void (*init) (void); //OF specific init function
-    void (*process_dio) (); //DIO processing callback (acc. to OF0 spec, chpt 5)
+    void (*process_dio) (void); //DIO processing callback (acc. to OF0 spec, chpt 5)
 } rpl_of_t;
 
 typedef struct {