[krbdev.mit.edu #6045] loadfuncs doesn't declare krb5_*_error_message functions

[asanka@mit.edu via RT]

The loadfuncs library does not declare function pointer types for the
following krb5 exports:

krb5_get_error_message()
krb5_free_error_message()
krb5_clear_error_message()

This patch adds these missing declarations.  Note that the patch is
against the /pismere/athena/util/loadfuncs/ directory in the Pismere
repository.