Fix build error when compiling with mingw

uintptr_t must be defined when including valgrind.h, just move it as
last include solves the problem.

https://bugzilla.gnome.org/show_bug.cgi?id=796283

gobject/gclosure.c

@@ -22,7 +22,6 @@
 
 #include "config.h"
 
-#include "../glib/valgrind.h"
 #include <string.h>
 
 #include <ffi.h>
@@ -35,6 +34,7 @@
 #include "gvaluetypes.h"
 #include "gtype-private.h"
 
+#include "../glib/valgrind.h"
 
 /**
  * SECTION:gclosure