bpo-40611: Adds MAP_POPULATE to the mmap module (GH-20061)

MAP_POPULATE constant has now been added to the list of exported
mmap module flags. 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index a3e22d0..8a60db1 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1574,6 +1574,9 @@
     setint(dict, "MAP_ANON", MAP_ANONYMOUS);
     setint(dict, "MAP_ANONYMOUS", MAP_ANONYMOUS);
 #endif
+#ifdef MAP_POPULATE
+    setint(dict, "MAP_POPULATE", MAP_POPULATE);
+#endif
 
     setint(dict, "PAGESIZE", (long)my_getpagesize());