Use OS file separator

Change-Id: I46b15bc1c1b4f2300a6fd98fe16c755da4910e7a
diff --git a/manifest_xml.py b/manifest_xml.py
index 73e3496..4f0eb64 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -164,7 +164,7 @@
     try:
       if os.path.lexists(self.manifestFile):
         os.remove(self.manifestFile)
-      os.symlink('manifests/%s' % name, self.manifestFile)
+      os.symlink(os.path.join('manifests', name), self.manifestFile)
     except OSError as e:
       raise ManifestParseError('cannot link manifest %s: %s' % (name, str(e)))