radv: Free syncobj with multiple imports.

Otherwise we can leak the old syncobj.

Fixes: eaa56eab "radv: initial support for shared semaphores (v2)"
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 917d3b43)