add --HEAD to fwknop formula to compile from git

This modification to the fwknop formula allows you to compile directly
from the upstream github repository's master branch. While not recommended,
this may allow you to build fwknop that includes fixes that have not been
packaged in to a versioned release.

The reason for this modification stems directly from an issue on the
fwknop project[1] where it would not compile on OS X Mavericks (10.9).
While there was a fix merged, it has yet to make it in to a versioned
release of fwknop.

[1] https://github.com/mrash/fwknop/issues/108