core: Make pkt_size and pkt_pad size_t

Fix bad signedness comparison:

```
core.c: In function ‘mctp_binding_set_tx_enabled’:
core.c:630:25: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
  630 |   if (binding->pkt_size < MCTP_PACKET_SIZE(MCTP_BTU)) {
      |                         ^
cc1: all warnings being treated as errors
```

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Idb91b16efa43526ea8e86fff666d0151f2987247
diff --git a/libmctp.h b/libmctp.h
index 00bce29..7aa24a2 100644
--- a/libmctp.h
+++ b/libmctp.h
@@ -102,8 +102,8 @@
 	uint8_t version;
 	struct mctp_bus *bus;
 	struct mctp *mctp;
-	int pkt_size;
-	int pkt_pad;
+	size_t pkt_size;
+	size_t pkt_pad;
 	int (*start)(struct mctp_binding *binding);
 	int (*tx)(struct mctp_binding *binding, struct mctp_pktbuf *pkt);
 	mctp_rx_fn control_rx;