[core] Fix modernize-use-override errors

As reported by clang-tidy-8.

platform/default/src/mbgl/map/map_snapshotter.cpp

@@ -108,7 +108,7 @@ public:
         : renderer(std::make_unique<util::Thread<SnapshotterRenderer>>(
               "Snapshotter", size, pixelRatio, std::move(localFontFamily))) {}
 
-    ~SnapshotterRendererFrontend() = default;
+    ~SnapshotterRendererFrontend() override = default;
 
     void reset() override { renderer->actor().invoke(&SnapshotterRenderer::reset); }
 

platform/glfw/glfw_view.cpp

@@ -43,7 +43,7 @@
 
 class SnapshotObserver final : public mbgl::MapSnapshotterObserver {
 public:
-    ~SnapshotObserver() = default;
+    ~SnapshotObserver() override = default;
     void onDidFinishLoadingStyle() override {
         if (didFinishLoadingStyleCallback) {
             didFinishLoadingStyleCallback();