Silence GCC 8 cast-function-type warnings

On Windows, casting between unrelated function types is fine and
sometimes even necessary, so just use an intermediate cast to
(void (*) (void)) to silence the warning as described in [0].

[0] https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html

Closes https://github.com/curl/curl/pull/2860

lib/formdata.c

@@ -39,6 +39,7 @@
 #include "sendf.h"
 #include "strdup.h"
 #include "rand.h"
+#include "warnless.h"
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
 #include "curl_memory.h"
@@ -878,7 +879,8 @@ CURLcode Curl_getformdata(struct Curl_easy *data,
                compatibility: use of "-" pseudo file name should be avoided. */
             result = curl_mime_data_cb(part, (curl_off_t) -1,
                                        (curl_read_callback) fread,
-                                       (curl_seek_callback) fseek,
+                                       CURLX_FUNCTION_CAST(curl_seek_callback,
+                                                           fseek),
                                        NULL, (void *) stdin);
           }
           else