pcm/Export: add (dummy) method Cancel()

We'll have some code for it soon.

src/pcm/PcmExport.hxx

@@ -149,6 +149,12 @@ struct PcmExport {
 	void Open(SampleFormat sample_format, unsigned channels,
 		  Params params);
 
+	/**
+	 * Reset the filter's state, e.g. drop/flush buffers.
+	 */
+	void Reset() {
+	}
+
 	/**
 	 * Calculate the size of one output frame.
 	 */