libnfsidmap: Use public plugin header file if available
    
    libnfsidmap was merged with nfs-utils in 2.1.2-rc7.
    
    We used private internal interface but required internal functions
    were dropped in nfs-utils-2.3.1. We need to use newly available public
    interface.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>