eap-radius: Increase Acct-Session-ID string buffer

As the startup timestamp needs 10 characters, we only have left 4 characters
for the IKE_SA unique identifier. This is insufficient when having 10000 IKE_SAs
or more established, resulting in non-unique session identifiers.

Fixes #889.

src/libcharon/plugins/eap_radius/eap_radius_accounting.c

@@ -99,7 +99,7 @@ typedef struct {
 	/** IKE_SA identifier this entry is stored under */
 	ike_sa_id_t *id;
 	/** RADIUS accounting session ID */
-	char sid[16];
+	char sid[24];
 	/** number of sent/received octets/packets */
 	struct {
 		u_int64_t sent;