add private constructor for util class

Author: oldratlee

src/main/java/com/alibaba/dcm/DnsCacheManipulator.java

@@ -170,4 +170,7 @@ public static void clearDnsCache() {
             throw new DnsCacheManipulatorException("Fail to clearDnsCache, cause: " + e.toString(), e);
         }
     }
+
+    private DnsCacheManipulator() {
+    }
 }

src/main/java/com/alibaba/dcm/internal/InetAddressCacheUtil.java

@@ -174,4 +174,7 @@ public static void clearInetAddressCache() throws NoSuchFieldException, IllegalA
             getCacheFiledOfInetAddress$CacheEntry().clear();
         }
     }
+
+    private InetAddressCacheUtil() {
+    }
 }