Silence warning about unused variable (#2765)

* Removed unused variable

I was getting a warning about this.

* Silence unused variable otherwise

dlib/media/ffmpeg_utils.h

@@ -1456,6 +1456,7 @@ namespace dlib
 
             const int ret = av_image_fill_arrays(src_pointers, src_linesizes, (uint8_t*)img.begin(), f.pixfmt(), f.width(), f.height(), 1);
             DLIB_ASSERT(ret == imgsize, "av_image_fill_arrays()  error : " << details::get_av_error(ret));
+            (void)imgsize;
             (void)ret;
             
             av_image_copy(f.get_frame().data,