RFR: Adds support for Gradle's incremental annotation processing

[Taylor Wicksell]

This change adds support for Gradle's [incremental annotation processing](https://docs.gradle.org/current/userguide/java_plugin.html#sec:incremental_annotation_processing). Without this support, developers using the JMH annotation processor in their Gradle projects will see their compilation time increased due to Gradle invalidating its incremental compilation cache.

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Commit messages:
 - Adds support for Gradle's incremental annotation processing

Changes: https://git.openjdk.java.net/jmh/pull/43/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmh&pr=43&range=00
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmh/pull/43.diff
  Fetch: git fetch https://git.openjdk.java.net/jmh pull/43/head:pull/43

PR: https://git.openjdk.java.net/jmh/pull/43