BUILD: Fix installation without samba
    
    winbindplugindir is defined only when BUILD_SAMBA is on. Also the file
    doesn't exist when BUILD_SAMBA is off, so installation will fail.
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 13adcd07000ba3ca1422c6ee863df17d70e2b14c)