core/pktbuf: Add next pointer to struct mctp_pktbuf

We will want to implement singly-linked queues of packets, so add a next
pointer.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/libmctp.h b/libmctp.h
index 5400788..4c45e51 100644
--- a/libmctp.h
+++ b/libmctp.h
@@ -43,6 +43,7 @@
 	unsigned char	data[MCTP_PKTBUF_SIZE];
 	uint8_t		start, end;
 	uint8_t		mctp_hdr_off;
+	struct mctp_pktbuf *next;
 };
 
 struct mctp_pktbuf *mctp_pktbuf_alloc(uint8_t len);