Rename doc.css to doc.css.in.

As we'll need to copy it to buck-out/gen anyway, this will save some
future trouble.

Change-Id: I91cfadd15d6642a8dfaf73b6243411e9492781ca

Documentation/asciidoc.defs

diff --git a/Documentation/asciidoc.defs b/Documentation/asciidoc.defs
index 4d00c50..6581dd7 100644
--- a/Documentation/asciidoc.defs
+++ b/Documentation/asciidoc.defs
@@ -34,7 +34,7 @@
   for attribute in attributes:
     asciidoc.extend(['-a', attribute])
   asciidoc.append('$SRCS')
-  newsrcs = ["doc.css"]
+  newsrcs = [":doc.css"]
   for src in srcs:
     fn = src
     # We have two cases: regular source files and generated files.