add private constructor for Slf4jLoggingHelpers.

Author: zhumin8

oauth2_http/java/com/google/auth/oauth2/Slf4jLoggingHelpers.java

@@ -177,4 +177,6 @@ private static String bytesToHex(byte[] hash) {
     }
     return hexString.toString();
   }
+
+  private Slf4jLoggingHelpers() {}
 }