set RANAP msgb headroom to 512 (times two)

With a headroom of 256, the SGSN crashes with:

  msgb(0x6e3b90): Not enough headroom msgb_push (256 < 264)

I'm not perfectly sure what amount of headroom is strictly necessary. The only
reason to pick 512 is that it is twice the amount of 256.

Change-Id: I9a193846902a0477af0873f78283c4f2bedaf5dd

src/ranap_common.c

@@ -291,7 +291,7 @@ char *ranap_cause_str(const RANAP_Cause_t *cause)
 
 static struct msgb *ranap_msgb_alloc(void)
 {
-	return msgb_alloc_headroom(1024+256, 256, "RANAP Tx");
+	return msgb_alloc_headroom(1024+512, 512, "RANAP Tx");
 }
 
 static struct msgb *_ranap_gen_msg(RANAP_RANAP_PDU_t *pdu)