fix a few warnings in the Java binding

src/runtime/java/org/grammaticalframework/pgf/ExprProb.java

@@ -9,6 +9,7 @@ public class ExprProb {
 		this.prob = prob;
 	}
 	
+	@SuppressWarnings("unused")
 	private static ExprProb mkExprProb(Pool pool, PGF gr, long expr, double prob) {
 		return new ExprProb(new Expr(pool, gr, expr), prob);
 	}

src/runtime/java/org/grammaticalframework/pgf/PGFError.java

@@ -1,6 +1,8 @@
 package org.grammaticalframework.pgf;
 
 public class PGFError extends RuntimeException {
+	private static final long serialVersionUID = -5098784200043861938L;
+
 	public PGFError(String message) {
 		super(message);
 	}

src/runtime/java/org/grammaticalframework/pgf/ParseError.java

@@ -1,6 +1,8 @@
 package org.grammaticalframework.pgf;
 
 public class ParseError extends Exception {
+	private static final long serialVersionUID = -6086991674218306569L;
+
 	public ParseError(String token) {
 		super(token);
 	}