crimson/os: mark dtor of interface class "virtual"

we use a smart_ptr in Seastore to manage the life cycle of
`TransactionManager` which is in turn a subclass
`ExtentCallbackInterface`, so should declare the dtor of latter as
virtual.

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/crimson/os/seastore/segment_cleaner.h

@@ -37,6 +37,7 @@ public:
   /// Callback interface for querying and operating on segments
   class ExtentCallbackInterface {
   public:
+    virtual ~ExtentCallbackInterface() = default;
     /**
      * get_next_dirty_extent
      *