[xmp] Fix data type in coreaudio driver

Misty De Meo warns that there's no uint8 in OSX 10.6 and earlier. Using
plain unsigned char instead.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>

src/sound_coreaudio.c

@@ -21,7 +21,7 @@ static AudioUnit au;
  */
 
 static int paused;
-static uint8 *buffer;
+static unsigned char *buffer;
 static int buffer_len;
 static int buf_write_pos;
 static int buf_read_pos;