Fix previous commit

Cygwin needs to be specifically disabled in the "supported_platforms"
array, since indicating "unix" support seems to also imply "cygwin"
support, according to the semantics described in the LuaRocks docs:

> If any positive entry exists, then at least one entry must match
> positively to the currently running platform.
> ...
> Note that, as of 0.6, Cygwin under Windows defines {"unix", "cygwin"}
> but not "windows".

-- https://github.com/luarocks/luarocks/wiki/Rockspec-format

rockspec.in

@@ -1,6 +1,6 @@
 package = "gumbo"
 version = "%VERSION%.%VSUFFIX%-1"
-supported_platforms = {"unix", "windows"}
+supported_platforms = {"unix", "windows", "!cygwin"}
 
 description = {
     summary = "HTML5 parser and DOM library",