define a constant for the max length of called party BCD IE

According to TS 24.008 10.5.4.7 the called party BSC number IE
has a maximum length of 43 octets.

This length is assumed inside osmo-hlr with a magic number:
uint8_t msisdn_enc[43]; /* TODO use constant; TS 24.008 10.5.4.7 */

This change makes libosmocore provide a constant which osmo-hlr can use.

Change-Id: Ia0bf6ceadcac38a8c75d166402b54058e5c6c6d4
diff --git a/include/osmocom/gsm/gsup.h b/include/osmocom/gsm/gsup.h
index 1a8a3b2..a7fa820 100644
--- a/include/osmocom/gsm/gsup.h
+++ b/include/osmocom/gsm/gsup.h
@@ -52,6 +52,7 @@
 #define OSMO_GSUP_MAX_NUM_AUTH_INFO		5
 /*! Maximum number of octets encoding MSISDN in BCD format */
 #define OSMO_GSUP_MAX_MSISDN_LEN		9
+#define OSMO_GSUP_MAX_CALLED_PARTY_BCD_LEN	43 /* TS 24.008 10.5.4.7 */
 
 #define OSMO_GSUP_PDP_TYPE_SIZE			2