manifest: include Python.h before standard headers

Python.h should be included before any standard headers according to the
python docs: https://docs.python.org/2/c-api/intro.html#include-files

mercurial/manifest.c

@@ -6,12 +6,12 b''
  * This software may be used and distributed according to the terms of
  * the GNU General Public License, incorporated herein by reference.
  */
+#include <Python.h>
+
 #include <assert.h>
 #include <string.h>
 #include <stdlib.h>
 
-#include <Python.h>
-
 /* VC9 doesn't include bool and lacks stdbool.h based on my searching */
 #ifdef _MSC_VER
 #define true 1