Add missing empty

Author: reuk

src/pointer-analysis/value_set.h

@@ -80,6 +80,7 @@ class value_sett
     const_iterator cend() const { return data.cend(); }
 
     size_t size() const { return data.size(); }
+    bool empty() const { return data.empty(); }
 
     objectt &operator[](unsigned i) { return data[i]; }