RFR: 8314268: Missing include in assembler_riscv.hpp

[Robbin Ehn]

Hello, please consider.

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Commit messages:
 - Added a missing include

Changes: https://git.openjdk.org/jdk/pull/15285/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314268
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15285.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15285/head:pull/15285

PR: https://git.openjdk.org/jdk/pull/15285