AppleMD: Add destructors to make compilers happy

src/AppleMessageDigestImpl.cc

@@ -49,6 +49,7 @@ template<size_t dlen,
 class MessageDigestBase : public MessageDigestImpl {
 public:
   MessageDigestBase() { reset(); }
+  virtual ~MessageDigestBase() {}
 
   virtual size_t getDigestLength() const {
     return dlen;

src/AppleMessageDigestImpl.h

@@ -45,6 +45,7 @@ namespace aria2 {
 
 class MessageDigestImpl {
 public:
+  virtual ~MessageDigestImpl() {}
   static SharedHandle<MessageDigestImpl> sha1();
   static SharedHandle<MessageDigestImpl> create(const std::string& hashType);