Make constants final in  DepQBF4J

Firstly, I am finding the solver very handy and fast for my applications especially the JNI interface is very useful, so thanks for that!

While using the java interface through JNI I found the constants declared in DepQBF4J.java were not final.
The problem with this is a switch statement requires a constant expression. At the moment one cannot do something like:
```java
 public boolean isSatisfiable()  {
        byte exitCode  = DepQBF4J.sat();
        switch (exitCode){
            case DepQBF4J.RESULT_SAT:
                return true;
            case DepQBF4J.RESULT_UNSAT:
                return false;
            default:
            .....
}
```
The changes just add a final modifier to the constants so they can be used as such.