Adding LLC-XID encoder / decoder and unit test

The lle-xid encoder/decoder is needed to encode and decode llc
xid parameter messages. We need this to exchange sndcp-parameters
(SNDCP-XID) and also simple parameters such as encryption IOVs

Change-Id: Ia06e4cb08bf9b48c2a4682606d1b1a91d19a9d37
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 0980413..fbf2930 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -229,6 +229,7 @@
     tests/oap/Makefile
     tests/gtphub/Makefile
     tests/mm_auth/Makefile
+    tests/xid/Makefile
     doc/Makefile
     doc/examples/Makefile
     Makefile)