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/test_common.c b/src/tests/test_common.c
index a79d5f5..5a37e7e 100644
--- a/src/tests/test_common.c
+++ b/src/tests/test_common.c
@@ -37,7 +37,7 @@
 #include <osmocom/core/talloc.h>
 #include <osmocom/core/logging.h>
 
-#include "hnbgw.h"
+#include <osmocom/iuh/hnbgw.h>
 
 void *talloc_asn1_ctx;