gsm_04_08: Make gsm48_secure_channel available globally

Can't declare it in gsm_04_08.h tough ... gsm_cbfn is defined in
gsm_data.h and trying to include that in gsm_04_08 just creates a
huge mess.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index 7d4f595..65d795a 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -140,8 +140,8 @@
 	                                  struct gsm_security_operation);
 }
 
-static int gsm48_secure_channel(struct gsm_subscriber_connection *conn, int key_seq,
-				gsm_cbfn *cb, void *cb_data)
+int gsm48_secure_channel(struct gsm_subscriber_connection *conn, int key_seq,
+                         gsm_cbfn *cb, void *cb_data)
 {
 	struct gsm_network *net = conn->bts->network;
 	struct gsm_subscriber *subscr = conn->subscr;