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/Makefile.am b/include/openbsc/Makefile.am
index 25709f1..89e0338 100644
--- a/include/openbsc/Makefile.am
+++ b/include/openbsc/Makefile.am
@@ -49,9 +49,9 @@
 	handover.h \
 	handover_decision.h \
 	ipaccess.h \
-	iu.h \
 	iucs.h \
 	iucs_ranap.h \
+	iu_dummy.h \
 	meas_feed.h \
 	meas_rep.h \
 	mgcp.h \