I don’t know of any libraries that do this. I can think of a couple workarounds though:
You could try to get the actual HTML output from Hakyll before it gets converted into a string (or Text or ByteString or whatever). Then you could write your own renderer (or maybe reuse an existing one) that produces minified HTML.
If Hakyll doesn’t expose the HTML before rendering it, you could parse the string output and then pass it through the above mentioned renderer.