Fixed error in gmtime example

gmtime and gmtime_r take a time_t pointer, so have to store the value
of time(NULL) on the stack.

Signed-off-by: Ole Herman Schumacher Elgesem <oleherman93@gmail.com>

cpp/ql/src/Security/CWE/CWE-676/PotentiallyDangerousFunction.c

@@ -1,12 +1,14 @@
 // BAD: using gmtime
 int is_morning_bad() {
-    struct tm *now = gmtime(time(NULL));
+    const time_t now_seconds = time(NULL);
+    struct tm *now = gmtime(&now_seconds);
     return (now->tm_hour < 12);
 }
 
 // GOOD: using gmtime_r
 int is_morning_good() {
+    const time_t now_seconds = time(NULL);
     struct tm now;
-    gmtime_r(time(NULL), &now);
+    gmtime_r(&now_seconds, &now);
     return (now.tm_hour < 12);
 }