typecheck-gcc: fix _curl_is_slist_info

Info values starting with CURLINFO_SOCKET expect a curl_socket_t, not a
curl_slist argument.

This fixes the following GCC warning when building the examples with
--enable-optimize:

../../include/curl/typecheck-gcc.h:126:42: warning: call to
‘_curl_easy_getinfo_err_curl_slist’ declared with attribute warning:
curl_easy_getinfo expects a pointer to 'struct curl_slist *' for this
info [enabled by default]
sendrecv.c:90:11: note: in expansion of macro ‘curl_easy_getinfo’
res = curl_easy_getinfo(curl, CURLINFO_ACTIVESOCKET, &sockfd);

Closes https://github.com/curl/curl/pull/1447