libheif/plugins/encoder_jpeg.cc: fix libjpeg build

[ffontaine]

Fix the following libjpeg build failure raised since version 1.17.0 and https://github.com/strukturag/libheif/commit/ebd13a20b8b7f1964939642b08b662ef7e483f39 because third argument of jpeg_mem_dest is defined as size_t* on libjpeg instead of unsigned long* on jpeg-turbo:

/home/buildroot/autobuild/instance-3/output-1/build/libheif-1.17.5/libheif/plugins/encoder_jpeg.cc: In function 'heif_error jpeg_encode_image(void*, const heif_image*, heif_image_input_class)': /home/buildroot/autobuild/instance-3/output-1/build/libheif-1.17.5/libheif/plugins/encoder_jpeg.cc:366:37: error: invalid conversion from 'long unsigned int*' to 'size_t*' {aka 'unsigned int*'} [-fpermissive]
  366 |   jpeg_mem_dest(&cinfo, &outbuffer, &outlength);
      |                                     ^~~~~~~~~~
      |                                     |
      |                                     long unsigned int*

Fix #1008 and #1086

Fixes:

• http://autobuild.buildroot.org/results/8ca909564c8dabe28ad08c96ebbc04b25592e727