Copy styles.css to the output directory

Follows whatwg/html#5536.

Author: domenic

build.sh

@@ -520,6 +520,7 @@ Disallow: /review-drafts/" > "$HTML_OUTPUT/robots.txt"
     cp -p  "$HTML_SOURCE/404.html" "$HTML_OUTPUT"
     cp -p "$HTML_SOURCE/link-fixup.js" "$HTML_OUTPUT"
     cp -p "$HTML_SOURCE/html-dfn.js" "$HTML_OUTPUT"
+    cp -p "$HTML_SOURCE/styles.css" "$HTML_OUTPUT"
     cp -pR "$HTML_SOURCE/fonts" "$HTML_OUTPUT"
     cp -pR "$HTML_SOURCE/images" "$HTML_OUTPUT"
     cp -pR "$HTML_SOURCE/demos" "$HTML_OUTPUT"