si13: Use the correct pseudo length for the SI13 message

The GSM04.08 Section 10.5.2.19 specifies the L2 Pseudo Length
and the length does not include rest octets, so we will need
to use a zero for the length.

The patch is coming from Dieter Spaar.

openbsc/src/system_information.c

@@ -451,7 +451,8 @@ static int generate_si13(u_int8_t *output, struct gsm_bts *bts)
 	if (ret < 0)
 		return ret;
 
-	si13->header.l2_plen = ret & 0xff;
+	/* length is coded in bit 2 an up */
+	si13->header.l2_plen = 0x01;
 
 	return sizeof (*si13) + ret;
 }