RFR: 8230758: ZGC: Add missing precompiled include and fix friend declaration

[Per Liden]

Looks good.

/Per

On 9/9/19 12:13 PM, Stefan Karlsson wrote:
> Hi all,
> 
> Please review this small patch to add a missing include of 
> precompiled.hpp and fix a friend declaration to use class instead of 
> struct.
> 
> https://cr.openjdk.java.net/~stefank/8230758/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8230758
> 
> Thanks,
> StefanK