obs-frontend-api: Use virtual destructor (fix memory leak)

Ensures data within the derived object is automatically freed.

UI/obs-frontend-api/obs-frontend-internal.hpp

@@ -6,6 +6,7 @@
 #include <string>
 
 struct obs_frontend_callbacks {
+	virtual ~obs_frontend_callbacks() {}
 	virtual void *obs_frontend_get_main_window(void)=0;
 	virtual void *obs_frontend_get_main_window_handle(void)=0;