Fix GenStubs in langtools for build-infra builds

The file langtools/make/tools/genstubs/GenStubs.java is not compiling:

/home/kohair/hgrepos/jdk8/jdk8-build-infra-work/langtools/make/tools/genstubs/GenStubs.java:204: error: incompatible types; no instance(s) of type variable(s) K,V exist so that Map<K,V> conforms to DocCommentTable
            tree.docComments = Collections.emptyMap();
                                                   ^
  required: DocCommentTable
  found: <K,V>Map<K,V>
  where K,V are type-variables:
    K extends Object declared in method <K,V>emptyMap()
    V extends Object declared in method <K,V>emptyMap()
1 error