Allocate SCCP user primitives with headroom

In I19cb83302aaa404ab1a2d92e6f2aec43d0380426 I set the headroom of
msgb's for SCCP User primitives to zero, assuming that we wouldn't
need any headroom in those primitives.

According to pespin, osmo-msc however needs this headroom:

DLSCCP <002e> sccp_user.c:156 Delivering N-CONNECT.indication to SCCP User 'OsmoMSC-A'
msgb(0xadfba0): Not enough headroom msgb_push (0 < 264)

So let's make sure the new SCCP User primitives are allocated with the
same headroom, just like before I19cb83302aaa404ab1a2d92e6f2aec43d0380426.

Change-Id: I92d7648f8ffd034341e2f12aa79dd3d16ec3a98d

src/sccp_helpers.c

@@ -34,7 +34,7 @@
 #include "sccp_internal.h"
 
 #define SCU_MSG_SIZE		2048
-#define SCU_MSG_HEADROOM	0
+#define SCU_MSG_HEADROOM	512
 
 static struct msgb *scu_msgb_alloc(const char *name)
 {