RFR: JDK-8211792: Fix misplaced BarrierSet forward declarations

[Roman Kennke]

In templateTable.hpp there's a forward declaration for BarrierSet
missing, while in oop.hpp we have a gratituous one.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8211792
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8211792/webrev.00/

Good? Also, passes as trivial?

Thanks,
Roman