Trivial MacOS compiler warning

[Aleksey Shipilev]

On 02/01/2018 11:22 AM, Lennart Börjeson wrote:
> That patch fixed both problems, thank you!
> 
> I now see one more warning which seems to be in Shenandoah:
> 
> /Users/lennartb/RaT/openJDK/shenandoah-jdk10/src/hotspot/share/gc/shenandoah/shenandoahStringDedup.cpp:157:5: warning: delete called on 'ShenandoahStrDedupTableCleanupTask' that is abstract but has non-virtual destructor [-Wdelete-non-virtual-dtor]
>     delete _dedup_table_cleanup_task;
>     ^
> 1 warning generated.

OK, looks like we want ShenandoahStrDedupTableCleanupTask destructor to be virtual, even though
there are no destructors in any subclass of it:
 http://cr.openjdk.java.net/~shade/shenandoah/build/macos-1/webrev.03/

-Aleksey