move libiu to osmo-iuh/libosmo-ranap

Remove libiu here, use the functions from libosmo-ranap instead, by applying
the ranap_ / RANAP_ prefix.

Corresponding change-id in osmo-iuh.git is I6a3f7ad15be03fb94689b4af6ccfa828c25f45c0

To be able to run the msc_vlr tests for RAN_UTRAN_IU without Iu client headers
available, add iu_dummy.h, containing mere function signatures that match
iu_dummy.c and a mostly empty struct ranap_ue_conn_ctx.

Make sure we can build with and without --enable-iu: include osmo-iuh headers
only with --enable-iu.

Change-Id: Ib8c4fcdb4766c5e575618b95ce16dce51063206b
diff --git a/include/openbsc/sgsn.h b/include/openbsc/sgsn.h
index 57b2978..f371dc6 100644
--- a/include/openbsc/sgsn.h
+++ b/include/openbsc/sgsn.h
@@ -112,7 +112,7 @@
 	} dcomp_v42bis;
 
 	struct {
-		enum nsap_addr_enc rab_assign_addr_enc;
+		int rab_assign_addr_enc;
 	} iu;
 };