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/tests/test_seq.c b/tests/test_seq.c
index e99dc94..8838acb 100644
--- a/tests/test_seq.c
+++ b/tests/test_seq.c
@@ -5,8 +5,8 @@
 #include <stdio.h>
 #include <string.h>
 
-#include <libmctp.h>
-
+#include "compiler.h"
+#include "libmctp.h"
 #include "test-utils.h"
 
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))