Add heimdal-krb5.pc file, with krb5.pc depending on it.

This makes it easier to install Heimdal and MIT kerberos on
the same system by just providing the heimdal-krb5.pc file;
the krb5.pc file is provided by both.

This is similar to what's done with heimdal-gssapi.pc/krb5-gssapi.pc.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>