libmctp: Introduce compiler.h

Define __unused here and include compiler.h in all the places we can
make use of it. Clean up some header ordering and include styles while
we're at it.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I33ddbdacbddbbf557dc02e42d8dde71a16e1cabe
diff --git a/utils/mctp-in.c b/utils/mctp-in.c
index c2792b6..eff4abf 100644
--- a/utils/mctp-in.c
+++ b/utils/mctp-in.c
@@ -1,5 +1,9 @@
 /* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
 
+#include "compiler.h"
+#include "libmctp.h"
+#include "libmctp-serial.h"
+
 #include <assert.h>
 #include <err.h>
 #include <stdio.h>
@@ -8,9 +12,6 @@
 #include <sys/poll.h>
 #include <sys/socket.h>
 
-#include "libmctp.h"
-#include "libmctp-serial.h"
-
 static void rx_message(uint8_t eid, void *data, void *msg, size_t len)
 {
 	ssize_t rc;