build: use _fseeki64() on Windows, drop detections

A recent update caused CMake builds to mis-detect this symbol on iOS.
Auto-detection also seems redundant given that it's a Windows-only
function and most Windows builds were already opted-in.

Drop detections and use it in all Windows builds with large file support
enabled.

Feature history:
- pririotizing for Windows: aaacd02466 #14678
- Windows opt-in cmake: 8e74c0729d #11950
- Windows opt-in: aa6c94c5bf #11944
- use in libcurl: 9c7165e96a #11918
- use in example: 817d1c0106

Regression from 8e34505776 #15164

Reported-by: Maarten Billemont
Fixes #15525
Closes #15526

lib/formdata.c

@@ -793,7 +793,7 @@ static CURLcode setname(curl_mimepart *part, const char *name, size_t len)
 /* wrap call to fseeko so it matches the calling convention of callback */
 static int fseeko_wrapper(void *stream, curl_off_t offset, int whence)
 {
-#if defined(HAVE__FSEEKI64)
+#if defined(_WIN32) && defined(USE_WIN32_LARGE_FILES)
   return _fseeki64(stream, (__int64)offset, whence);
 #elif defined(HAVE_FSEEKO) && defined(HAVE_DECL_FSEEKO)
   return fseeko(stream, (off_t)offset, whence);