[JBIDE-15256] Include source gen plugin for features into pluginManagement

Source generation plugins moved to features/pom.xml and removed from 
features/feature-id/pom.xml files. This PR should be applied after 
https://github.com/jbosstools/jbosstools-build/pull/107 pushed to
jbosstools-build.
 # f95ce1a (Rob Stryker, 2 weeks ago)