buffet: Add virtual StorageInterface::~StorageInterface()

Inherited classes were deleted as StorageInterface causing memory leaks.

BUG=brillo:1069
TEST=USE="clang asan" FEATURES=test emerge-gizmo buffet

Change-Id: I92c70b2d547821b0b59d48dc4c74ba0d898889ac
Reviewed-on: https://chromium-review.googlesource.com/271790
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>

buffet/storage_interface.h

diff --git a/buffet/storage_interface.h b/buffet/storage_interface.h
index d36e56d..c224513 100644
--- a/buffet/storage_interface.h
+++ b/buffet/storage_interface.h
@@ -15,6 +15,8 @@
 // the details of this storage behind an interface for test purposes.
 class StorageInterface {
  public:
+  virtual ~StorageInterface() = default;
+
   // Load the dictionary from storage. If it fails (e.g. the storage container
   // [file?] doesn't exist), then it returns empty unique_ptr (aka nullptr).
   virtual std::unique_ptr<base::DictionaryValue> Load() = 0;