login: Add nonstring attributes to struct utmp, struct utmpx [BZ #24899]

Commit 7532837d7b ("The
-Wstringop-truncation option new in GCC 8 detects common misuses")
added __attribute_nonstring__ to bits/utmp.h, but it did not update
the parallel bits/utmpx.h header.  In struct utmp, the nonstring
attribute for ut_id was missing.

bits/utmp.h

@@ -61,7 +61,8 @@ struct utmp
   pid_t ut_pid;			/* Process ID of login process.  */
   char ut_line[UT_LINESIZE]
     __attribute_nonstring__;	/* Devicename.  */
-  char ut_id[4];		/* Inittab ID.  */
+  char ut_id[4]
+    __attribute_nonstring__;	/* Inittab ID.  */
   char ut_user[UT_NAMESIZE]
     __attribute_nonstring__;	/* Username.  */
   char ut_host[UT_HOSTSIZE]