[mdns] Mark MDNSComponent as final

MDNSComponent is never subclassed. Marking it final enables the
compiler to devirtualize calls to the Component overrides (setup,
dump_config, get_setup_priority, on_shutdown).

esphome/components/mdns/mdns_component.h

@@ -40,7 +40,7 @@ struct MDNSService {
   FixedVector<MDNSTXTRecord> txt_records;
 };
 
-class MDNSComponent : public Component {
+class MDNSComponent final : public Component {
  public:
   void setup() override;
   void dump_config() override;