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

Specifically, nlr.h does.
diff --git a/py/objint_longlong.c b/py/objint_longlong.c
index 7d71c5a..d319e27 100644
--- a/py/objint_longlong.c
+++ b/py/objint_longlong.c
@@ -2,9 +2,9 @@
 #include <stdint.h>
 #include <string.h>
 
+#include "mpconfig.h"
 #include "nlr.h"
 #include "misc.h"
-#include "mpconfig.h"
 #include "qstr.h"
 #include "obj.h"
 #include "mpz.h"