fix: ensure liveupdate pubsubs exit (#5557)

Previously, the derived class (i.e. BttvLiveUpdates or SeventvEventAPI)
    would have their destructor ran before BasicPubSubManager called
    stop, meaning there was a time wherein messages could still flow
    through, attempting to call `onMessage` on a pure virtual, causing a
    crash.

src/providers/seventv/SeventvEventAPI.hpp

@@ -33,6 +33,8 @@ public:
                     std::chrono::milliseconds defaultHeartbeatInterval =
                         std::chrono::milliseconds(25000));
 
+    ~SeventvEventAPI() override;
+
     struct {
         Signal<seventv::eventapi::EmoteAddDispatch> emoteAdded;
         Signal<seventv::eventapi::EmoteUpdateDispatch> emoteUpdated;