RFR: 8294083: RISC-V: Minimal build failed with --disable-precompiled-headers

[Fei Yang]

The build failed due to undeclared 'java_lang_ref_Reference'.
Including the corresponding header (classfile/javaClasses.hpp) will fix it.
I see a similar issue was once fixed for aarch64: https://bugs.openjdk.org/browse/JDK-8257743

Testing: Minimal builds with this fix

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Commit messages:
 - 8294083: RISC-V: Minimal build failed with --disable-precompiled-headers

Changes: https://git.openjdk.org/jdk/pull/10368/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10368&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294083
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10368.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10368/head:pull/10368

PR: https://git.openjdk.org/jdk/pull/10368