Reuse compatibleVersion project property in benchmark build.gradle

[pderop]

When running the benchmark using the baseline (latest release of BlockHound) like this:

./gradlew --no-daemon benchmarks:jmh -PjmhTarget=baseline 

then the baseline version to use is hard coded in the benchmark build.gradle.
Now, for japicmp, we have introduced the baseline version in the gradle.properties (compatibleVersion).

Let's reuse this "compatibleVersion" project property from benchmark/gradle.properties, this will make a little bit easier the release process.

[pderop]

Since this PR is trivial, I'm going to merge it without any code review.