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-capture.h b/utils/mctp-capture.h
index 90feb66..e97d547 100644
--- a/utils/mctp-capture.h
+++ b/utils/mctp-capture.h
@@ -5,6 +5,7 @@
 
 #include "config.h"
 
+#include "compiler.h"
 #include "libmctp.h"
 
 #include <sys/types.h>
@@ -19,8 +20,6 @@
 #define CAPTURE_LINKTYPE_FIRST	147
 #define CAPTURE_LINKTYPE_LAST	162
 
-#define __unused __attribute__((unused))
-
 struct capture {
 	const char	*path;
 	int		linktype;