mscsplit: gsm_network_init(): add explicit root talloc ctx

Decouple the root talloc context from libbsc's global talloc_bsc_ctx.

This allows to define the root talloc ctx from a main() scope, which in turn
helps decouple libmsc from libbsc.

Change-Id: I92f6b47b1eeea2e8f3fba66f25d7e708e5659f8a
diff --git a/openbsc/tests/channel/channel_test.c b/openbsc/tests/channel/channel_test.c
index 5674607..924d763 100644
--- a/openbsc/tests/channel/channel_test.c
+++ b/openbsc/tests/channel/channel_test.c
@@ -68,7 +68,7 @@
 	printf("Testing the gsm_subscriber chan logic\n");
 
 	/* Create a dummy network */
-	network = gsm_network_init(1, 1, NULL);
+	network = gsm_network_init(tall_bsc_ctx, 1, 1, NULL);
 	if (!network)
 		exit(1);
 	bts = gsm_bts_alloc(network);