Fix serial javac warnings in ArrayPrefixHelpers

/w/lthudson/hudson-data/jobs/jdk8-lt-jdk-build/workspace/jdk/src/share/classes/java/util/ArrayPrefixHelpers.java:100: warning: [serial] serializable class java.util.ArrayPrefixHelpers.CumulateTask has no definition of serialVersionUID
    static final class CumulateTask<T> extends CountedCompleter<Void> {
                 ^
/w/lthudson/hudson-data/jobs/jdk8-lt-jdk-build/workspace/jdk/src/share/classes/java/util/ArrayPrefixHelpers.java:250: warning: [serial] serializable class java.util.ArrayPrefixHelpers.LongCumulateTask has no definition of serialVersionUID
    static final class LongCumulateTask extends CountedCompleter<Void> {
                 ^
/w/lthudson/hudson-data/jobs/jdk8-lt-jdk-build/workspace/jdk/src/share/classes/java/util/ArrayPrefixHelpers.java:399: warning: [serial] serializable class java.util.ArrayPrefixHelpers.DoubleCumulateTask has no definition of serialVersionUID
    static final class DoubleCumulateTask extends CountedCompleter<Void> {
                 ^
/w/lthudson/hudson-data/jobs/jdk8-lt-jdk-build/workspace/jdk/src/share/classes/java/util/ArrayPrefixHelpers.java:548: warning: [serial] serializable class java.util.ArrayPrefixHelpers.IntCumulateTask has no definition of serialVersionUID
    static final class IntCumulateTask extends CountedCompleter<Void> {
                 ^