Trade: add a defaulted deinlined destructor to AbstractSceneConverter.

Otherwise I suppose it'd be generated inline at every call site, including
the vtable. Do not want.

src/Magnum/Trade/AbstractSceneConverter.cpp

@@ -99,6 +99,8 @@ AbstractSceneConverter::AbstractSceneConverter(PluginManager::Manager<AbstractSc
 
 AbstractSceneConverter::AbstractSceneConverter(PluginManager::AbstractManager& manager, const Containers::StringView& plugin): PluginManager::AbstractManagingPlugin<AbstractSceneConverter>{manager, plugin} {}
 
+AbstractSceneConverter::~AbstractSceneConverter() = default;
+
 SceneConverterFeatures AbstractSceneConverter::features() const {
     const SceneConverterFeatures features = doFeatures();
     CORRADE_ASSERT(features, "Trade::AbstractSceneConverter::features(): implementation reported no features", {});

src/Magnum/Trade/AbstractSceneConverter.h

@@ -329,6 +329,8 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
            header. */
         explicit AbstractSceneConverter(PluginManager::AbstractManager& manager, const Containers::StringView& plugin);
 
+        ~AbstractSceneConverter();
+
         /** @brief Features supported by this converter */
         SceneConverterFeatures features() const;