[misc] Silence compiler warning...

If someone tries to set the type to unknown he should know
what he is doing, just silence the warning.
diff --git a/openbsc/src/gsm_data.c b/openbsc/src/gsm_data.c
index a4b3212..a10edad 100644
--- a/openbsc/src/gsm_data.c
+++ b/openbsc/src/gsm_data.c
@@ -534,6 +534,8 @@
 		break;
 	case GSM_BTS_TYPE_BS11:
 		break;
+	case GSM_BTS_TYPE_UNKNOWN:
+		break;
 	}
 
 	return 0;