RFR: Fix Windows builds after oopMapStubGenerator change

[Aleksey Shipilev]

On Mon, 27 Sep 2021 11:27:26 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Turns out, MSVC still compiles this compilation unit, unless we expose the PCH include. (Sighs)
> Seen in GHA here: https://github.com/shipilev/loom/runs/3716995428?check_suite_focus=true -- and reproduced locally.
> 
> Additional testing:
>  - [x] Local Windows 10 (MSVC 2019) builds now pass

All right, Windows builds pass: 
https://github.com/shipilev/loom/runs/3719701023?check_suite_focus=true
https://github.com/shipilev/loom/runs/3719701058?check_suite_focus=true

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PR: https://git.openjdk.java.net/loom/pull/68