password_inc variable not being used

From the includes/password.inc file's comments:

* An alternative or custom version of this password hashing API may be
* used by setting the variable password_inc to the name of the PHP file
* containing replacement user_hash_password(), user_check_password(), and
* user_needs_new_hash() functions.

This patch modifies bakery to use the variable as it is used in user.install