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/.gitignore b/openbsc/.gitignore
index 0a2965d..518a960 100644
--- a/openbsc/.gitignore
+++ b/openbsc/.gitignore
@@ -81,6 +81,7 @@
 tests/oap/oap_test
 tests/gtphub/gtphub_test
 tests/mm_auth/mm_auth_test
+tests/xid/xid_test
 
 tests/atconfig
 tests/atlocal