enlarge the GSUP message headroom

Make room for (more) arbitrary IPA headers, like longer IPA names as configured
by the user.

Change-Id: I7d86f2dadcae29fe1550ea2c9773394ab31a837b
diff --git a/src/gsup_server.c b/src/gsup_server.c
index 053151f..4819ea4 100644
--- a/src/gsup_server.c
+++ b/src/gsup_server.c
@@ -37,7 +37,7 @@
 
 struct msgb *osmo_gsup_msgb_alloc(const char *label)
 {
-	struct msgb *msg = msgb_alloc_headroom(1024+16, 16, label);
+	struct msgb *msg = msgb_alloc_headroom(1024+512, 512, label);
 	OSMO_ASSERT(msg);
 	return msg;
 }