fix build with javadoc in java 8

Hi

I had some troubles building the simulator on my machine (latest ubuntu with latest openjdk).
This patch fixes one of the problems I encountered.

Fwiw, I had another problem with https://github.com/domsj/kinetic-java/blob/1a6eb187aa18cdb3187c0e48d6b0335e5fbb09b4/kinetic-common/pom.xml#L58 in which the ${project.parent.basedir} somehow resulted in null, but I don't know what the appropriate fix there should be. I just hardcoded the needed path in there.

Kind regards,
Jan