RFR: Fix Windows builds after oopMapStubGenerator change

[Aleksey Shipilev]

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

-------------

Commit messages:
 - MSVC barfs when precompiled.hpp is not included in .cpp that is "if 0"-ed. Bad MSVC. Bad.

Changes: https://git.openjdk.java.net/loom/pull/68/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=68&range=00
  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/loom/pull/68.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/68/head:pull/68

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