Unbreak compilation with CHECK_STRUCTS.

Commit 3963aea4f4 modified the ‘buffer’
structure, but didn’t adapt the hash.

* src/pdumper.c (dump_buffer): Update buffer hash.

src/pdumper.c

@@ -2692,7 +2692,7 @@ 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_5DC36DBD42
+#if CHECK_STRUCTS && !defined HASH_buffer_EE36B4292E
 # error "buffer changed. See CHECK_STRUCTS comment in config.h."
 #endif
   struct buffer munged_buffer = *in_buffer;