Add a few override declarations to avoid clang++ warnings.

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/mp3inputstream.hh

@@ -44,7 +44,7 @@ public:
   ~MP3InputStream();
 
   Error   open (const std::string& filename);
-  Error   read_frames (std::vector<float>& samples, size_t count);
+  Error   read_frames (std::vector<float>& samples, size_t count) override;
   void    close();
 
   int     bit_depth() const override;

src/rawoutputstream.hh

@@ -45,7 +45,7 @@ public:
 
   Error open (const std::string& filename, const RawFormat& format);
   Error write_frames (const std::vector<float>& frames) override;
-  Error close();
+  Error close() override;
 };
 
 #endif /* AUDIOWMARK_RAW_OUTPUT_STREAM_HH */

src/sfinputstream.hh

@@ -43,11 +43,11 @@ public:
   ~SFInputStream();
 
   Error               open (const std::string& filename);
-  Error               read_frames (std::vector<float>& samples, size_t count);
+  Error               read_frames (std::vector<float>& samples, size_t count) override;
   void                close();
 
   int
-  n_channels() const
+  n_channels() const override
   {
     return m_n_channels;
   }

src/stdoutwavoutputstream.hh

@@ -44,7 +44,7 @@ public:
 
   Error open (int n_channels, int sample_rate, int bit_depth, size_t n_frames);
   Error write_frames (const std::vector<float>& frames) override;
-  Error close();
+  Error close() override;
   int  sample_rate() const override;
   int  bit_depth() const override;
   int  n_channels() const override;