libc/localtime: fix timegm return error when tz with isdst

testcase:
set TZ CEST
time_t t = time(NULL);
struct tm *ts = localtime(&t);
t = timegm(ts);
t will return (time_t)-1;

Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

libs/libc/time/lib_localtime.c

@@ -2584,5 +2584,10 @@ time_t mktime(FAR struct tm *tmp)
 
 time_t timegm(FAR struct tm *tmp)
 {
+  if (tmp != NULL)
+    {
+      tmp->tm_isdst = 0;
+    }
+
   return time1(tmp, gmtsub, 0L);
 }