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/include/osmocom/rua/Makefile.am b/include/osmocom/rua/Makefile.am
index 49e2150..307f123 100644
--- a/include/osmocom/rua/Makefile.am
+++ b/include/osmocom/rua/Makefile.am
@@ -1,5 +1,5 @@
 noinst_HEADERS = \
-	rua_common.h rua_ies_defs.h \
+	rua_common.h rua_ies_defs.h rua_msg_factory.h \
 	RUA_Ansi-41-IDNNS.h \
 	RUA_Cause.h \
 	RUA_CauseMisc.h \