RFR: 8297941: Add override modifier in space.hpp

[Albert Mingkun Yang]

Simple patch generated using `clang-tidy`.

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Commit messages:
 - s1-override

Changes: https://git.openjdk.org/jdk/pull/11451/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11451&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297941
  Stats: 38 lines in 1 file changed: 0 ins; 0 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/11451.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11451/head:pull/11451

PR: https://git.openjdk.org/jdk/pull/11451