fix prototype of write_data() function used by curl

maybe this works, but I don't want it to suddenly stop working
some day just because curl guys made their library more strict.

src/helpers.h

@@ -75,7 +75,7 @@ const Buffer &ShowTag(const std::string &);
 const std::basic_string<my_char_t> &Scroller(const std::string &, size_t, size_t &);
 
 #ifdef HAVE_CURL_CURL_H
-size_t write_data(char *, size_t, size_t, std::string);
+size_t write_data(char *, size_t, size_t, void *);
 #endif
 
 #endif