Added cast to curl_read_callback functions to mute warnings.

xs_curl.h

@@ -125,11 +125,11 @@ xs_dict *xs_http_request(const char *method, const char *url,
 
     /* store response headers here */
     curl_easy_setopt(curl, CURLOPT_HEADERDATA,     &response);
-    curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, _header_callback);
+    curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, (curl_read_callback) _header_callback);
 
     struct _payload_data ipd = { NULL, 0, 0 };
     curl_easy_setopt(curl, CURLOPT_WRITEDATA,      &ipd);
-    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION,  _data_callback);
+    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, (curl_read_callback) _data_callback);
 
     if (strcmp(method, "POST") == 0 || strcmp(method, "PUT") == 0) {
         CURLoption curl_method = method[1] == 'O' ? CURLOPT_POST : CURLOPT_UPLOAD;
@@ -147,7 +147,7 @@ xs_dict *xs_http_request(const char *method, const char *url,
             pd.offset = 0;
 
             curl_easy_setopt(curl, CURLOPT_READDATA,     &pd);
-            curl_easy_setopt(curl, CURLOPT_READFUNCTION, _post_callback);
+            curl_easy_setopt(curl, CURLOPT_READFUNCTION, (curl_read_callback) _post_callback);
         }
     }
 
@@ -232,7 +232,7 @@ int xs_smtp_request(const char *url, const char *user, const char *pass,
     curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, rcpt);
 
     curl_easy_setopt(curl, CURLOPT_READDATA, &pd);
-    curl_easy_setopt(curl, CURLOPT_READFUNCTION, _post_callback);
+    curl_easy_setopt(curl, CURLOPT_READFUNCTION, (curl_read_callback) _post_callback);
     curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
 
     res = curl_easy_perform(curl);