selftests/timers: Add missing error code assignment before test

In order to work, the 'err' return value has to be updated otherwise the
test can never be true.

Signed-off-by: Christophe JAILLET <[e-mail]>
Acked-by: John Stultz <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/timers/posix_timers.c

···
 	else if (which == ITIMER_REAL)
 		idle_loop();
 
-	gettimeofday(&end, NULL);
+	err = gettimeofday(&end, NULL);
 	if (err < 0) {
 		perror("Can't call gettimeofday()\n");
 		return -1;
···
 
 	user_loop();
 
-	gettimeofday(&end, NULL);
+	err = gettimeofday(&end, NULL);
 	if (err < 0) {
 		perror("Can't call gettimeofday()\n");
 		return -1;