decoder/flac: pass void pointer to flac_convert()

That function diverts into various bit formats; it doesn't need a
typed pointer.

src/decoder/_flac_common.c

@@ -290,9 +290,9 @@ flac_convert_8(int8_t *dest,
 			*dest++ = buf[c_chan][position];
 }
 
-static void flac_convert(unsigned char *dest,
-			 unsigned int num_channels,
-			 unsigned sample_format,
+static void
+flac_convert(void *dest,
+	     unsigned int num_channels, unsigned sample_format,
 	     const FLAC__int32 *const buf[],
 	     unsigned int position, unsigned int end)
 {