mkstemp: permission 0666 to 0600

https://man7.org/linux/man-pages/man3/mkstemp.3.html
remove the read/write permission of other users for temp file

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

libs/libc/stdlib/lib_mkstemp.c

@@ -66,7 +66,7 @@ int mkstemp(FAR char *path_template)
 
   if (path)
     {
-      ret = open(path, O_RDWR | O_CREAT | O_EXCL, 0666);
+      ret = open(path, O_RDWR | O_CREAT | O_EXCL, 0600);
     }
 
   return ret;