import gprs_gsup_message.[ch] from openbsc as gsup.[ch]

Move those routines from OpenBSC to libosmogsm, so they can be
re-used from other programs.  I think it was a mistake to add them only
inside the openbsc repository in the first place.  We need to pay more
attention to this in the future.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 8bd0dbf..e0d9dcb 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -309,5 +309,8 @@
 osmo_apn_to_str;
 osmo_apn_from_str;
 
+osmo_gsup_encode;
+osmo_gsup_decode;
+
 local: *;
 };