Fix subchan_demux_test compiler warning on some gcc versions

In commit Ia082b9fddf03d02afd007825a1588a3ef0dbedae we switched
from 'uint8_t *' to 'const ubit_t *' - change the test case
accordingly.

Change-Id: I7d24b891a21e6e485738ddf4a302583579b012d5
diff --git a/tests/subchan_demux/subchan_demux_test.c b/tests/subchan_demux/subchan_demux_test.c
index 2a9a678..9afd765 100644
--- a/tests/subchan_demux/subchan_demux_test.c
+++ b/tests/subchan_demux/subchan_demux_test.c
@@ -199,7 +199,7 @@
 
 
 
-static int data_cb(struct subch_demux *demux, int ch, uint8_t *data, int len, void *p)
+static int data_cb(struct subch_demux *demux, int ch, const ubit_t *data, int len, void *p)
 {
 	printf("DATA_CB Channel(%d): %s\n",
 		ch, osmo_hexdump(data, len));