event.c: Fix -Wmissing-prototypes warnings.

src/libespeak-ng/event.c

 static node *tail = NULL;
 static int node_counter = 0;
 static espeak_ng_STATUS push(void *data);
-static void *pop();
-static void init();
+static void *pop(void);
+static void init(void);
 static void *polling_thread(void *);
 
 void event_set_callback(t_espeak_callback *SynthCallback)

src/libespeak-ng/event.h

 void event_set_callback(t_espeak_callback *cb);
 
 // Clear any pending event.
-espeak_ng_STATUS event_clear_all();
+espeak_ng_STATUS event_clear_all(void);
 
 // Declare a future event
 espeak_ng_STATUS event_declare(espeak_EVENT *event);
 
 // Terminate the event component.
 // Last function to be called.
-void event_terminate();
+void event_terminate(void);
 
 // general functions
 void clock_gettime2(struct timespec *ts);