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

Specifically, nlr.h does.
diff --git a/py/objclosure.c b/py/objclosure.c
index 09371b0..ca7c537 100644
--- a/py/objclosure.c
+++ b/py/objclosure.c
@@ -1,8 +1,8 @@
 #include <string.h>
 
+#include "mpconfig.h"
 #include "nlr.h"
 #include "misc.h"
-#include "mpconfig.h"
 #include "qstr.h"
 #include "obj.h"
 #include "objtuple.h"