Fix test failure on Windows

I confess to having no idea whether the code will work correctly; my
only know test case (https://github.com/hynek/argon2_cffi) fails to
build on Windows[*] so I cannot run check-manifest on it.

[*] setup.py sdist fails due to some path being of the form
    "c:\\path\\_ffi_build.py:ffi" instead of "path/build.py:ffi_variable"

check_manifest.py

@@ -19,6 +19,7 @@
 import fnmatch
 import locale
 import os
+import posixpath
 import re
 import shutil
 import subprocess
@@ -270,7 +271,7 @@ def add_prefix_to_each(prefix, filelist):
         ['foo/bar/a', 'foo/bar/b', 'foo/bar/c/d']
 
     """
-    return [os.path.join(prefix, name) for name in filelist]
+    return [posixpath.join(prefix, name) for name in filelist]
 
 
 class VCS(object):