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/tests/hnb-test.c b/src/tests/hnb-test.c
index 2c41199..d338077 100644
--- a/src/tests/hnb-test.c
+++ b/src/tests/hnb-test.c
@@ -54,7 +54,7 @@
 #include "hnb-test-layers.h"
 #include <osmocom/hnbap/hnbap_common.h>
 #include <osmocom/hnbap/hnbap_ies_defs.h>
-#include "rua_msg_factory.h"
+#include <osmocom/rua/rua_msg_factory.h>
 #include "asn1helpers.h"
 #include <osmocom/ranap/iu_helpers.h>
 #include "test_common.h"