[krbdev.mit.edu #6046] NIM: dynimport should include krb5_*_error_message() functions

[asanka@mit.edu via RT]

The /src/windows/identity/plugins/common/dynimport.{c,h} files are used
by the NIM Kerberos v5 plug-ins for run-time dynamic linking.  They
currently do not declare or import the following functions:

krb5_get_error_message()
krb5_free_error_message()
krb5_clear_error_message()

This patch adds declarations and definitions required for locating these
functions in dynimport.*, and is generated against the
src/windows/identity directory in the SVN trunk.

Depends on ticket #6045 (loadfuncs doesn't declare krb5_*_error_message
functions)