MINOR: Use annotationProcessor instead of compile for JMH annotation processor

This fixes the following Gradle warning:

> The following annotation processors were detected on the compile classpath:
> 'org.openjdk.jmh.generators.BenchmarkProcessor'. Detecting annotation processors
> on the compile classpath is deprecated and Gradle 5.0 will ignore them. Please add
> them to the annotation processor path instead. If you did not intend to use annotation
> processors, you can use the '-proc:none' compiler argument to ignore them.

With this change, the warning went away and `./jmh-benchmarks/jmh.sh` continues
to work.

Author: Ismael Juma <ismael@juma.me.uk>

Reviewers: Dong Lin <lindong28@gmail.com>

Closes #5602 from ijuma/annotation-processor

build.gradle

@@ -1198,7 +1198,7 @@ project(':jmh-benchmarks') {
     compile project(':clients')
     compile project(':streams')
     compile libs.jmhCore
-    compile libs.jmhGeneratorAnnProcess
+    annotationProcessor libs.jmhGeneratorAnnProcess
     compile libs.jmhCoreBenchmarks
   }