C++: Fix 'implicit use of this'.

Author: MathiasVP

cpp/ql/lib/experimental/semmle/code/cpp/dataflow/ProductFlow.qll

@@ -54,15 +54,15 @@ module ProductFlow {
      * dataflow graph when the flow state is `state`.
      */
     predicate isBarrier1(DataFlow::Node node, DataFlow::FlowState state) {
-      isBarrier1(node) and state = ""
+      this.isBarrier1(node) and state = ""
     }
 
     /**
      * Holds if data flow through `node` is prohibited through the second projection of the product
      * dataflow graph when the flow state is `state`.
      */
     predicate isBarrier2(DataFlow::Node node, DataFlow::FlowState state) {
-      isBarrier2(node) and state = ""
+      this.isBarrier2(node) and state = ""
     }
 
     /**