Fix "OSError: [WinError 6] The handle is invalid" failures

Summary:
# Context
Attempt to fix: https://fb.workplace.com/groups/buck2windows/permalink/932995478572365/

Similar to this change from scottcao D55267096

Reviewed By: krallin

Differential Revision: D55515411

fbshipit-source-id: 51e22a511f8a3c922019ca1ecf140d1d3c1ca491

cxx/tools/makefile_to_dep_file.py

@@ -88,7 +88,7 @@ def process_dep_file(args):
     Expects the src dep file to be the first argument, dst dep file to be the
     second argument, and the command to follow.
     """
-    ret = subprocess.call(args[2:])
+    ret = subprocess.call(args[2:], stdin=subprocess.DEVNULL)
     if ret == 0:
         rewrite_dep_file(args[0], args[1])
     sys.exit(ret)