RFR: 8291238: JDK can't be built without G1

[Calvin Cheung]

Please review this simple change for fixing build failure without G1.

The fix is by adding `#if INCLUDE_G1GC ... #endif` around some G1 specific code.

Passed builds-tier1 with G1GC disabled.

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Commit messages:
 - 8291238: JDK can't be built without G1

Changes: https://git.openjdk.org/jdk/pull/9800/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291238
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9800.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9800/head:pull/9800

PR: https://git.openjdk.org/jdk/pull/9800