output: pass the music chunk pointer as void*, not char*

The meaning of the chunk depends on the audio format; don't suggest a
specific format by declaring the pointer as "char*", pass "void*"
instead.

src/output/null_plugin.c

@@ -75,7 +75,7 @@ null_close(void *data)
 }
 
 static size_t
-null_play(void *data, G_GNUC_UNUSED const char *chunk, size_t size)
+null_play(void *data, G_GNUC_UNUSED const void *chunk, size_t size)
 {
 	struct null_data *nd = data;
 	Timer *timer = nd->timer;