C++: Fix sink argument types. In the map.cpp tests we have:

```
std::pair<std::pair<const char *, const char *>, const char *> m;
...
sink(m);
```
and we only had a sink type for `std::pair<const char *, const char *>`.

Author: MathiasVP

cpp/ql/test/library-tests/dataflow/taint-tests/map.cpp

@@ -8,7 +8,7 @@ char *source();
 void sink(char *);
 void sink(const char *);
 void sink(bool);
-void sink(std::pair<const char *, const char *>);
+template<typename T, typename U> void sink(std::pair<T, U>);
 void sink(std::map<const char *, const char *>);
 void sink(std::map<const char *, const char *>::iterator);
 void sink(std::unordered_map<const char *, const char *>);