[krbdev.mit.edu #7584] krb5_free_ktypes() needs a prototype in krb5.h

[Will Fiveash via RT]

krb5_get_permitted_enctypes()is public and outputs a ktypes list that
should be freed by krb5_free_ktypes() however krb5_free_ktypes() does
not have a prototype in krb5.h.