Fix jpg.cc for MinGW-w64 (libjxl#3821)

* Fix jpg.cc for MinGW-w64

Fix the following error when building with MinGW-w64:

```raw
lib/extras/enc/jpg.cc:277:34: error: cannot convert 'long unsigned int*' to 'size_t*' {aka 'long long
unsigned int*'}
  277 |   jpeg_mem_dest(&cinfo, &buffer, &size);
      |                                  ^~~~~
      |                                  |
      |                                  long unsigned int*
```

* Update jpg.cc

Use LIBJPEG_TURBO_VERSION to detect if libjpeg-turbo is used

* format

---------

Co-authored-by: Moritz Firsching <firsching@google.com>
(cherry picked from commit 0266f4d)

lib/extras/enc/jpg.cc

@@ -273,7 +273,11 @@ Status EncodeWithLibJpeg(const PackedImage& image, const JxlBasicInfo& info,
   cinfo.err = jpeg_std_error(&jerr);
   jpeg_create_compress(&cinfo);
   unsigned char* buffer = nullptr;
+#ifdef LIBJPEG_TURBO_VERSION
   unsigned long size = 0;  // NOLINT
+#else
+  size_t size = 0;  // NOLINT
+#endif
   jpeg_mem_dest(&cinfo, &buffer, &size);
   cinfo.image_width = image.xsize;
   cinfo.image_height = image.ysize;