const-correct ldap_err2string

[Ken_Lin@iris.com]

I'd like to propose we change the definition of
    char *ldap_err2string( int err );
as described in
ftp://ftp.ietf.org/internet-drafts/draft-ietf-ldapext-ldap-c-api-05.txt to
return a const char* like so:
    const char *ldap_err2string( int err );