[crac] RFR: 8364272: [CRaC] Make inclusion of crac.hpp in os_posix.cpp conditional

[Timofei Pushkin]

Just a small, mostly cosmetic enhancement.

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

Commit messages:
 - Make CRaC include conditional

Changes: https://git.openjdk.org/crac/pull/252/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=252&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364272
  Stats: 4 lines in 1 file changed: 3 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/crac/pull/252.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/252/head:pull/252

PR: https://git.openjdk.org/crac/pull/252