dirvote: Handling adding vote and signature if module is disabled

Both functions are used for directory request but they can only be used if the
running tor instance is a directory authority.

For this reason, make those symbols visible but hard assert() if they are
called when the module is disabled. This would mean we failed to safeguard the
entry point into the module.

Signed-off-by: David Goulet <dgoulet@torproject.org>