use C.struct, not _Ctype_struct

[mindscratch]

Using cgo with Go 1.12 fails

# github.com/andelf/go-curl
../src/github.com/andelf/go-curl/easy.go:408:22: identifier "_Ctype_struct_curl_slist" may conflict with identifiers generated by cgo

references:

• https://github.com/google/gopacket/pull/589
• https://github.com/weaveworks/weave/issues/3577
• https://github.com/golang/go/issues/28721