time: siginfo_recvd needs to be marked volatile

sig_atomic_t does not imply volatility, we must do it ourselves to avoid
caching of siginfo_recvd loads.

Sponsored by:	Klara, Inc.

usr.bin/time/time.c

@@ -54,7 +54,7 @@ static void showtime(FILE *, struct timespec *, struct timespec *,
 static void siginfo(int);
 static void usage(void) __dead2;
 
-static sig_atomic_t siginfo_recvd;
+static volatile sig_atomic_t siginfo_recvd;
 static char decimal_point;
 static struct timespec before_ts;
 static int hflag, pflag;