Fix 'static field should be accessed in a static way' warning

Change-Id: I9ea5bb37fa10c915ba39b294812f1c29d291fc09

src/builtin/PRED_open_4.java

diff --git a/src/builtin/PRED_open_4.java b/src/builtin/PRED_open_4.java
index 9a80fea..a9c2c7c 100644
--- a/src/builtin/PRED_open_4.java
+++ b/src/builtin/PRED_open_4.java
@@ -85,7 +85,7 @@
 		if (! file.exists())
 		    throw new ExistenceException(this, 1, "source_sink", a1, "");
 		PushbackReader in = 
-		    new PushbackReader(new BufferedReader(new FileReader(file)), engine.PUSHBACK_SIZE);
+		    new PushbackReader(new BufferedReader(new FileReader(file)), Prolog.PUSHBACK_SIZE);
 		streamObject = new JavaObjectTerm(in);
 		opts = new ListTerm(SYM_INPUT, opts);
 	    } else if (a2.equals(SYM_WRITE)) {