timeradd/timersub: cast result to void to get rid of warnings

previously:
timersub(&now, t, &diff);

warning: value computed is not used [-Wunused-value]

include/sys/time.h

@@ -43,10 +43,10 @@ int adjtime (const struct timeval *, struct timeval *);
 #define timerclear(t) ((t)->tv_sec = (t)->tv_usec = 0)
 #define timercmp(s,t,op) ((s)->tv_sec == (t)->tv_sec ? \
 	(s)->tv_usec op (t)->tv_usec : (s)->tv_sec op (t)->tv_sec)
-#define timeradd(s,t,a) ( (a)->tv_sec = (s)->tv_sec + (t)->tv_sec, \
+#define timeradd(s,t,a) (void) ( (a)->tv_sec = (s)->tv_sec + (t)->tv_sec, \
 	((a)->tv_usec = (s)->tv_usec + (t)->tv_usec) >= 1000000 && \
 	((a)->tv_usec -= 1000000, (a)->tv_sec++) )
-#define timersub(s,t,a) ( (a)->tv_sec = (s)->tv_sec - (t)->tv_sec, \
+#define timersub(s,t,a) (void) ( (a)->tv_sec = (s)->tv_sec - (t)->tv_sec, \
 	((a)->tv_usec = (s)->tv_usec - (t)->tv_usec) < 0 && \
 	((a)->tv_usec += 1000000, (a)->tv_sec--) )
 #endif