GSUP: Add support for RAND in SendAuthInfo.req

In the autentication re-sync case, we need not only the AUTS from
the MS/UE, but also the RAND that we sent as part of the failed
authentication challenge.
diff --git a/include/osmocom/gsm/gsup.h b/include/osmocom/gsm/gsup.h
index 523c15d..660ffd4 100644
--- a/include/osmocom/gsm/gsup.h
+++ b/include/osmocom/gsm/gsup.h
@@ -138,6 +138,7 @@
 	const uint8_t			*hlr_enc;
 	size_t				hlr_enc_len;
 	const uint8_t			*auts;
+	const uint8_t			*rand;
 	enum osmo_gsup_cn_domain	cn_domain;
 };