Add eclipse plugin to build.gradle to provide 'gradle eclipse' to regenerate Eclipse project files for adjusted JNA lib

Unfortunately the Eclipse .classpath does not seem to be the same on each machine, thus I added the eclipse-plugin to the gradle build file so that calling "gradle eclipse" will regenerate the files with the correct pathes.

In other projects we do not even check in the .classpath/.project/.settings files and simply describe this step as part of "start hacking" in the README...