Fix build constraints since windows does not depend on cgo

[gapra-msft]

The build constraints in ieproxy_unix and pac_unix are incorrect.

The windows implementation does not depend on cgo, only the darwin one does.

Based off the following logic this should be the build constraint on this file, otherwise Windows architectures with cgo disabled by default will not build correctly, resulting in an error like this

# github.com/mattn/go-ieproxy
../../../go/pkg/mod/github.com/mattn/[email protected]/ieproxy_windows.go:22:6: getConf redeclared in this block
	../../../go/pkg/mod/github.com/mattn/[email protected]/ieproxy_unix.go:6:6: other declaration of getConf
../../../go/pkg/mod/github.com/mattn/[email protected]/ieproxy_windows.go:28:6: reloadConf redeclared in this block
	../../../go/pkg/mod/github.com/mattn/[email protected]/ieproxy_unix.go:10:6: other declaration of reloadConf
../../../go/pkg/mod/github.com/mattn/[email protected]/ieproxy_windows.go:133:6: overrideEnvWithStaticProxy redeclared in this block
	../../../go/pkg/mod/github.com/mattn/[email protected]/ieproxy_unix.go:14:6: other declaration of overrideEnvWithStaticProxy
../../../go/pkg/mod/github.com/mattn/[email protected]/pac_windows.go:9:29: ProxyScriptConf.findProxyForURL redeclared in this block
	../../../go/pkg/mod/github.com/mattn/[email protected]/pac_unix.go:6:29: other declaration of findProxyForURL

!(windows || (darwin && cgo)) = !windows && !(darwin && cgo) = !windows && (!darwin || !cgo) = !windows,!darwin !cgo

//go:build !windows && (!darwin || !cgo)
// +build !windows,!darwin !cgo

I believe this should resolve #45 as well

[gesellix]

This would be great to be merged and released. I'm already using the patch in my projects, but I'd prefer not to use vendored modules :)

[gesellix]

@mattn do you think you could merge and release this change?

[mattn]

Thank you