add timestamp.cs to doclava to fix break (unknown dependencies for doc builds)

Change-Id: I3cc0675dfc6bd58c0eac3e4dc146816bcfe8f34b
diff --git a/res/assets/templates/timestamp.cs b/res/assets/templates/timestamp.cs
new file mode 100644
index 0000000..4bf502a
--- /dev/null
+++ b/res/assets/templates/timestamp.cs
@@ -0,0 +1 @@
+var BUILD_TIMESTAMP = "<?cs var:page.now ?>";