input/InputStream: make IsEOF() and IsAvailable() const

src/archive/plugins/ZzipArchivePlugin.cxx

@@ -110,7 +110,7 @@ public:
 	}
 
 	/* virtual methods from InputStream */
-	bool IsEOF() noexcept override;
+	bool IsEOF() const noexcept override;
 	size_t Read(std::unique_lock<Mutex> &lock,
 		    void *ptr, size_t size) override;
 	void Seek(std::unique_lock<Mutex> &lock, offset_type offset) override;
@@ -144,7 +144,7 @@ ZzipInputStream::Read(std::unique_lock<Mutex> &, void *ptr, size_t read_size)
 }
 
 bool
-ZzipInputStream::IsEOF() noexcept
+ZzipInputStream::IsEOF() const noexcept
 {
 	return offset_type(zzip_tell(file)) == size;
 }