async.c: fixed signature of dummy functions

The signature of several dummy functions that are defined to make
compilers happy that complain on empty modules was not standards
compliant. To avoid related compiler warnings, the formal argument
list of this function now is explicitly declared as void.

src/address.c

@@ -68,7 +68,7 @@ int coap_is_mcast(const coap_address_t *a) {
 #else /* !defined(WITH_CONTIKI) && !defined(WITH_LWIP) */
 
 /* make compilers happy that do not like empty modules */
-COAP_STATIC_INLINE void dummy()
+COAP_STATIC_INLINE void dummy(void)
 {
 }
 

src/async.c

@@ -106,5 +106,5 @@ coap_free_async(coap_async_state_t *s) {
 }
 
 #else
-void does_not_exist();	/* make some compilers happy */
+void does_not_exist(void);	/* make some compilers happy */
 #endif /* WITHOUT_ASYNC */

src/coap_time.c

@@ -129,7 +129,7 @@ coap_tick_t coap_ticks_from_rt_us(uint64_t t) {
 #else /* HAVE_TIME_H */
 
 /* make compilers happy that do not like empty modules */
-COAP_STATIC_INLINE void dummy()
+COAP_STATIC_INLINE void dummy(void)
 {
 }