libmsc/gsm_04_08: refactor require ciphering into an own function

Make it more readable.

Change-Id: I9e407f65b282e645feabe714f7f4c3e44fae21e9
diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index 2a64c53..34d42db 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -13,6 +13,7 @@
 #include <osmocom/core/stat_item.h>
 #include <osmocom/gsm/gsm48.h>
 #include <osmocom/crypt/auth.h>
+#include <osmocom/crypt/utran_cipher.h>
 
 #include <osmocom/mgcp_client/mgcp_client.h>