fix dsound_open() signature

The error was introduced with commit 33f5edd where
plugout_dsound.c was not updated.

I wonder why the Windows build pipelines did not fail.

plugout_dsound.c

@@ -30,11 +30,11 @@ static DWORD writeMax;       // Maximum amount we can write at once.
 static WAVEFORMATEX wfx;
 static DSBUFFERDESC dsbdesc;
 
-static long dsound_open(enum plugout_endian *endian, long rate, long *buffer_bytes, const char *const filename)
+static long dsound_open(enum plugout_endian *endian, long rate, long *buffer_bytes, const struct plugout_metadata metadata)
 {
 	HRESULT hr;
 
-	UNUSED(filename);
+	UNUSED(metadata);
 
 	*endian = PLUGOUT_ENDIAN_NATIVE;