build: move headers to include/osmocom/*

This came up while fixing 'make distcheck'; this is certainly not the easiest
way but it makes sense to have the headers in include/, like we do in openbsc.

The easy alternative might be to add -I$(top_srcdir)/src to src/Makefile.am.

Remove -I$(top_srcdir)/src from src/tests/Makefile.am, no longer needed.

Change-Id: I5a82e029dcdc4df0a60a31271a4883393fe59234
diff --git a/src/ranap_common_cn.c b/src/ranap_common_cn.c
index 2c80dd0..3736dce 100644
--- a/src/ranap_common_cn.c
+++ b/src/ranap_common_cn.c
@@ -29,7 +29,7 @@
 #include <osmocom/ranap/ranap_common_cn.h>
 #include <osmocom/ranap/ranap_ies_defs.h>
 
-#include "hnbgw.h"
+#include <osmocom/iuh/hnbgw.h>
 
 static int cn_ranap_rx_initiating_msg_co(void *ctx, RANAP_InitiatingMessage_t *imsg,
 					 ranap_message *message)