clang-tidy: add explicit deleted constructors

Found with cppcoreguidelines-special-member-functions

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/output/plugins/FifoOutputPlugin.cxx

@@ -49,6 +49,9 @@ public:
 		CloseFifo();
 	}
 
+	FifoOutput(const FifoOutput &) = delete;
+	FifoOutput &operator=(const FifoOutput &) = delete;
+
 	static AudioOutput *Create(EventLoop &,
 				   const ConfigBlock &block) {
 		return new FifoOutput(block);