Suppress EqualsWrongThing flagged by error prone

This fixes the build:

ERROR: /home/davido/projects/prolog-cafe/BUILD:46:1: Building \
  liblang.jar (48 source files) failed (Exit 1)
java/com/googlecode/prolog_cafe/lang/VariableTerm.java:193: error: \
  [EqualsWrongThing] Suspicious comparison between `val` and `this`
	if(val != this)
	       ^
    (see https://errorprone.info/bugpattern/EqualsWrongThing)

Change-Id: I17456d8ecea82862440b82993a99018bf9827987
diff --git a/src/lang/VariableTerm.java b/src/lang/VariableTerm.java
index 54b326b..8fb465e 100644
--- a/src/lang/VariableTerm.java
+++ b/src/lang/VariableTerm.java
@@ -14,6 +14,7 @@
  * @author Naoyuki Tamura (tamura@kobe-u.ac.jp)
  * @version 1.0
  */
+@SuppressWarnings("EqualsWrongThing")
 public class VariableTerm extends Term implements Undoable {
     /** Holds a term to which this variable is bound. Initial value is <code>this</code> (self-reference). */
     private Term val;