struct krb_principal lossage

[John Hawkinson]

Trying to build arla against MIT kerberos (or CNS kerberos)
fails becuase it depends on struct krb_principal, declared
in kth-kerberos' krb.h.

It'd be nice if rxkad/ didn't depend on it.
Of course, since rxkad/ came from kth-krb, I recognize this
might be a bit difficult.

Otherwise, I suppose autoconf should look and declare it
elsewhere...dunno what the best way to handle this is.

--jhawk