Fix != operator in preprocessor

Author: boriel

tests/functional/prepro90.bi

@@ -0,0 +1,9 @@
+
+#define MACRO YES
+
+#if MACRO != YES
+   WRONG
+#else
+   OK
+#endif
+

tests/functional/prepro90.out

@@ -0,0 +1,4 @@
+#line 1 "prepro90.bi"
+#line 7 "prepro90.bi"
+   OK
+#line 9 "prepro90.bi"

zxbpp/zxbpp.py

@@ -564,7 +564,7 @@ def p_expr(p):
 def p_exprne(p):
     """ expr : expr NE expr
     """
-    p[0] = '1' if p[1] != p[2] else '0'
+    p[0] = '1' if p[1] != p[3] else '0'
 
 
 def p_expreq(p):