wireshark: Drop DESTDIR from install path
    
    In 50078cfbcbf3c8 I've tried to fix distcheck but accidentally
    broke rpm build. The problem is that rpm build not only sets
    DESTDIR but also passes plugindir path. This results in double
    DESTDIR being in the plugin path, Drop one.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>