RFR: 8231251: ZGC: Fix ZHeap includes

[erik.osterlund at oracle.com]

Hi Per,

Looks amazing.

Thanks,
/Erik

On 09/19/2019 04:41 PM, Per Liden wrote:
> I noticed that the ZHeap includes needed a bit of a cleanup, as there 
> are many includes that are not longer needed, and a few are missing.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8231251
> Webrev: http://cr.openjdk.java.net/~pliden/8231251/webrev.0
>
> /Per