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

[Stefan Karlsson]

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