elf: Preserve _rtld_global layout for the release branch

Backporting commit 9d6577fdff
("elf: Introduce _dl_debug_change_state") removed the
_ns_debug member.  Keep it to preseve struct layout.

sysdeps/generic/ldsodefs.h

@@ -350,6 +350,8 @@ struct rtld_global
       size_t n_elements;
       void (*free) (void *);
     } _ns_unique_sym_table;
+    /* Keep track of changes to each namespace' list.  */
+    struct r_debug_extended _ns_debug_unused;
   } _dl_ns[DL_NNS];
   /* One higher than index of last used namespace.  */
   EXTERN size_t _dl_nns;