parser: use 'override' keyword in subclasses

src/parser/check_refs.cpp

@@ -57,7 +57,7 @@ public:
     ReferenceVisitor(size_t num_groups, const flat_set<string> &targets)
         : num_ids(num_groups), names(targets) {}
 
-    ~ReferenceVisitor();
+    ~ReferenceVisitor() override;
 
     void invalid_index(const char *component, unsigned id) {
         assert(component);

src/parser/prefilter.cpp

@@ -201,7 +201,7 @@ const ComponentSequence *findCapturingGroup(const Component *root,
 class PrefilterVisitor : public DefaultComponentVisitor {
 public:
     PrefilterVisitor(Component *c, const ParseMode &m) : root(c), mode(m) {}
-    ~PrefilterVisitor();
+    ~PrefilterVisitor() override;
 
     /** \brief Calls the visitor (recursively) on a new replacement component
      * we've just created. Takes care of freeing it if the sequence is itself

src/parser/shortcut_literal.cpp

@@ -64,7 +64,7 @@ namespace ue2 {
  */
 class ConstructLiteralVisitor : public ConstComponentVisitor {
 public:
-    ~ConstructLiteralVisitor();
+    ~ConstructLiteralVisitor() override;
 
     /** \brief Thrown if this component does not represent a literal. */
     struct NotLiteral {};

src/parser/unsupported.cpp

@@ -44,7 +44,7 @@ namespace ue2 {
  * an unsupported component. */
 class UnsupportedVisitor : public DefaultConstComponentVisitor {
 public:
-    ~UnsupportedVisitor();
+    ~UnsupportedVisitor() override;
     void pre(const ComponentAssertion &) override {
         throw ParseError("Zero-width assertions are not supported.");
     }