need to export kadm5_set_use_password_server

need to export kadm5_set_use_password_server

Need to export "kadm5_set_use_password_server" because it is used by
krb5kdc when USE_PASSWORD_SERVER is defined.  Since the Mac builds this
way we need it in the export list.  Unfortunately export lists don't get
preprocessed.  Note that I only added this one function.  The rest of the
password server functionality is still conditionalized around
USE_PASSWORD_SERVER.

Hopefully we can rip all this code out when Apple starts using the db plugin
interface for their password server integration.

ticket: new
target_version: 1.5

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@17977 dc483132-0cff-0310-8789-dd5450dbe970