UI: Fix da_push_back taking a wrong type of item

A pointer to `OBSSource` was passed to an argument that expect a pointer
to `obs_source_t *`.
Since the current implementation of a class `OBSSource` has a member
`obs_source_t *obj` at first and the class does not have any virtual
member functions, most implementation should work correctly.

UI/api-interface.cpp

@@ -117,8 +117,8 @@ struct OBSStudioAPI : obs_frontend_callbacks {
 		struct obs_frontend_source_list *sources) override
 	{
 		for (int i = 0; i < main->ui->transitions->count(); i++) {
-			OBSSource tr = main->ui->transitions->itemData(i)
-					       .value<OBSSource>();
+			obs_source_t *tr = main->ui->transitions->itemData(i)
+						   .value<OBSSource>();
 
 			if (!tr)
 				continue;