fix MNCC memory leak

after passing the mncc structure (contained in msgb) to the mncc layer,
we have to release its memory.  This leak was discovered as a direct result of
using talloc.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index 56db654..c77268e 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -3809,6 +3809,7 @@
 			if (net->mncc_recv)
 				net->mncc_recv(net, mncc->msg_type, mncc);
 			work = 1; /* work done */
+			talloc_free(msg);
 		}
 
 	return work;