Update cpe for vulnerability scanners

Bug: http://b/201572953
Change-Id: I1ad40a619f07c73009e317cba7c8c7848779f5ee
diff --git a/METADATA b/METADATA
index b863408..d42a042 100644
--- a/METADATA
+++ b/METADATA
@@ -19,4 +19,7 @@
     month: 8
     day: 20
   }
+  security {
+      tag: "NVD-CPE2.3:cpe:/a:xmlsoft:libxml2:2.9.2:-"
+  }
 }