minor changes to make this a utility class (final, private constructor)

Former-commit-id: 0571f0eddc640925d5f5f2ffbdb873d082191ba9

dependency-check-core/src/main/java/org/owasp/dependencycheck/utils/Checksum.java

@@ -19,7 +19,13 @@ import java.util.logging.Logger;
  * @author Real's How To: http://www.rgagnon.com/javadetails/java-0416.html
  *
  */
-public class Checksum {
+public final class Checksum {
+
+    /**
+     * Private constructor for a utility class.
+     */
+    private Checksum() {
+    }
 
     /**
      * <p>Creates the cryptographic checksum of a given file using the specified
@@ -104,4 +110,4 @@ public class Checksum {
         }
         return hex.toString();
     }
-}
+}