CQ: Fix 3.13 upgrade error in the message store

The problem was due to a record being modified despite that
record being read from disk. To avoid any potential issues
the record was put back to the way it used to be, only now
we have 3 'unused' fields.

Author: Loïc Hoguin

deps/rabbit/src/rabbit_msg_store.erl

@@ -129,7 +129,7 @@
         }).
 
 -record(file_summary,
-        {file, valid_total_size, file_size, locked}).
+        {file, valid_total_size, unused1, unused2, file_size, locked, unused3}).
 
 -record(gc_state,
         { dir,