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_astlpc.c b/tests/test_astlpc.c
index 32c221f..054eda1 100644
--- a/tests/test_astlpc.c
+++ b/tests/test_astlpc.c
@@ -12,9 +12,10 @@
 #define pr_fmt(x) "test: " x
 #endif
 
+#include "compiler.h"
+#include "container_of.h"
 #include "libmctp-astlpc.h"
 #include "libmctp-log.h"
-#include "container_of.h"
 
 #ifdef NDEBUG
 #undef NDEBUG
@@ -149,8 +150,6 @@
 	.lpc_write = mctp_astlpc_mmio_lpc_write,
 };
 
-#define __unused __attribute__((unused))
-
 static void rx_message(uint8_t eid __unused, void *data __unused, void *msg,
 		       size_t len)
 {