Fix gcc sign conversion warnings in event.c.

src/libespeak-ng/event.c

 	ts->tv_nsec = tv.tv_usec*1000;
 }
 
-void add_time_in_ms(struct timespec *ts, int time_in_ms)
+void add_time_in_ms(struct timespec *ts, uint64_t time_in_ms)
 {
 	if (!ts)
 		return;
 
-	uint64_t t_ns = (uint64_t)ts->tv_nsec + 1000000 * (uint64_t)time_in_ms;
+	uint64_t t_ns = (uint64_t)ts->tv_nsec + 1000000 * time_in_ms;
 	while (t_ns >= ONE_BILLION) {
 		ts->tv_sec += 1;
 		t_ns -= ONE_BILLION;

src/libespeak-ng/wave.h

 
 // general functions
 extern void clock_gettime2(struct timespec *ts);
-extern void add_time_in_ms(struct timespec *ts, int time_in_ms);
+extern void add_time_in_ms(struct timespec *ts, uint64_t time_in_ms);
 
 // for tests
 extern void *wave_test_get_write_buffer();