[Storage] Fix build error by forward declaring StorageReferenceInternal

Co-authored-by: AustinBenoit <22805659+AustinBenoit@users.noreply.github.com>

Author: google-labs-jules[bot]

storage/src/include/firebase/storage/list_result.h

@@ -24,6 +24,7 @@ class StorageReference;
 
 namespace internal {
 class StorageListResultInternal;
+class StorageReferenceInternal;
 }  // namespace internal
 
 /// @brief Represents the result of calling List on a StorageReference.