Unbreak builds with CHECK_STRUCTS.

* src/pdumper.c (dump_buffer): Fix hash for 'struct buffer'.  The
recent changes to that structure where commits
8f3091defb and
0bd4661941, both of which just affected
comments.

src/pdumper.c

@@ -2748,7 +2748,7 @@ dump_hash_table (struct dump_context *ctx,
 static dump_off
 dump_buffer (struct dump_context *ctx, const struct buffer *in_buffer)
 {
-#if CHECK_STRUCTS && !defined HASH_buffer_85D317CE74
+#if CHECK_STRUCTS && !defined HASH_buffer_9E96D7C4B4
 # error "buffer changed. See CHECK_STRUCTS comment in config.h."
 #endif
   struct buffer munged_buffer = *in_buffer;