py, unix: Make "mpconfig.h" be first included, as other headers depend on it.

Specifically, nlr.h does.
diff --git a/py/objobject.c b/py/objobject.c
index 74a4926..62fd0c4 100644
--- a/py/objobject.c
+++ b/py/objobject.c
@@ -1,8 +1,8 @@
 #include <stdlib.h>
 
+#include "mpconfig.h"
 #include "nlr.h"
 #include "misc.h"
-#include "mpconfig.h"
 #include "qstr.h"
 #include "obj.h"
 #include "runtime0.h"