Histogram class need to be used by cl, so expose it to public

Author: ericsson-peiwuyang

src/main/java/com/cisco/trex/stateful/model/stats/LatencyPortHist.java

@@ -6,7 +6,7 @@
 
 public class LatencyPortHist {
 
-    private static class Histogram{
+    public static class Histogram{
         @SerializedName("key")
         public Integer key;