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/gprs_sgsn.h b/include/openbsc/gprs_sgsn.h
index 4e49c08..57995e0 100644
--- a/include/openbsc/gprs_sgsn.h
+++ b/include/openbsc/gprs_sgsn.h
@@ -117,7 +117,7 @@
 	uint16_t pdp_status;
 };
 
-struct ue_conn_ctx;
+struct ranap_ue_conn_ctx;
 
 /* According to TS 03.60, Table 5: SGSN MM and PDP Contexts */
 /* Extended by 3GPP TS 23.060, Table 6: SGSN MM and PDP Contexts */
@@ -159,7 +159,7 @@
 		/* CSG Subscription Data */
 		/* LIPA Allowed */
 		/* Voice Support Match Indicator */
-		struct ue_conn_ctx	*ue_ctx;
+		struct ranap_ue_conn_ctx	*ue_ctx;
 		struct service_info	service;
 	} iu;
 	/* VLR number */