database: Make _parse_message_id static once again.

We had exposed this to the internal implementation for a short time,
(only while we had the silly code fetching In-Reply-To values from
message files instead of from the database). Make this private again
as it should be.

lib/database.cc

@@ -317,7 +317,7 @@ skip_space_and_comments (const char **str)
  * Returns a newly talloc'ed string belonging to 'ctx'.
  *
  * Returns NULL if there is any error parsing the message-id. */
-char *
+static char *
 _parse_message_id (void *ctx, const char *message_id, const char **next)
 {
     const char *s, *end;

lib/notmuch-private.h

@@ -147,9 +147,6 @@ typedef enum _notmuch_private_status {
 const char *
 _find_prefix (const char *name);
 
-char *
-_parse_message_id (void *ctx, const char *message_id, const char **next);
-
 /* thread.cc */
 
 notmuch_thread_t *