[crac] RFR: Fix clang compilation error

[Jan Kratochvil]

src/hotspot/share/gc/g1/g1CollectedHeap.hpp:1023:16: error: 'finish_collection' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  virtual void finish_collection() {
               ^
src/hotspot/share/gc/shared/collectedHeap.hpp:396:16: note: overridden virtual function is here
  virtual void finish_collection() {}
               ^

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

Commit messages:
 - Fix clang compilation error

Changes: https://git.openjdk.org/crac/pull/117/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=117&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/crac/pull/117.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/117/head:pull/117

PR: https://git.openjdk.org/crac/pull/117