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/hnbgw_vty.c b/src/hnbgw_vty.c
index 1673c0c..2e3d1e9 100644
--- a/src/hnbgw_vty.c
+++ b/src/hnbgw_vty.c
@@ -22,8 +22,8 @@
 
 #include <osmocom/iuh/vty.h>
 
-#include "hnbgw.h"
-#include "context_map.h"
+#include <osmocom/iuh/hnbgw.h>
+#include <osmocom/iuh/context_map.h>
 
 static void *tall_hnb_ctx = NULL;
 static struct hnb_gw *g_hnb_gw = NULL;