add 'libosmotrau' for TRAU/RTP related code

This new library is intended to include everything related to
interfacing actual voice channels either via E1 or via RTP.

The first module in the library is osmo_rtp, based on the ortp library.
diff --git a/include/osmocom/Makefile.am b/include/osmocom/Makefile.am
index f9d5e7c..a8574ba 100644
--- a/include/osmocom/Makefile.am
+++ b/include/osmocom/Makefile.am
@@ -1 +1 @@
-SUBDIRS=abis
+SUBDIRS=abis trau