Declare unused parameters

When building with global CFLAGS=-Wunused-parameter the output from the
build looks terrible.  This commit explicitly declares unused variables
to prevent triggering -Wunused-parameter

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/conf.c

@@ -1562,6 +1562,8 @@ static int conf_iwatch_read(int fd)
 
 static void conf_cb(uev_t *w, void *arg, int events)
 {
+	(void)arg;
+
 	if (UEV_ERROR == events) {
 		dbg("%s(): iwatch socket %d invalid.", __func__, w->fd);
 		return;