Fix compiler warning in ipa-pwd-extop
    
    cast const error message to non-const char*. I tried to make errMesg a
    const char* but it gets passed down to slapi_send_ldap_result() which
    accepts a char*.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>