sctp: sctp_output.c failed to compile when VLIB_BUFFER_TRACE_TRAJECTORY is enabled

Fixed a typo in sctp_push_header(). It was inherited from tcp_output.c

Change-Id: I810fcb4c24cfd3d54f15da72a5184cfc4df24592
Signed-off-by: Steven <sluong@cisco.com>
diff --git a/src/vnet/sctp/sctp_output.c b/src/vnet/sctp/sctp_output.c
index 5e64ca7..b735127 100644
--- a/src/vnet/sctp/sctp_output.c
+++ b/src/vnet/sctp/sctp_output.c
@@ -1381,7 +1381,7 @@
 
   sctp_push_hdr_i (sctp_conn, b, SCTP_STATE_ESTABLISHED);
 
-  sctp_trajectory_add_start (b0, 3);
+  sctp_trajectory_add_start (b, 3);
 
   return 0;
 }