nostrdb/blocks: add total_size

Fix this mistake that we have with ndb_notes where we don't know the
total size of the object

Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/nostrdb.h

@@ -469,6 +469,7 @@ int ndb_parse_content(unsigned char *buf, int buf_size,
 
 // BLOCKS
 enum ndb_block_type ndb_block_type(struct ndb_blocks *blocks);
+size_t ndb_blocks_total_size(struct ndb_blocks *blocks);