media: ffmpeg: Fix test_averror not compiling.

Broke in 03e1b667 ("media/ffmpeg: use bindgen-common.sh").

BUG=None
TEST=cargo test -p ffmpeg

Change-Id: I7e0bb5f9b41f785d63a92099130d183600a7985b
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3907424
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Auto-Submit: Tatsuyuki Ishi <ishitatsuyuki@google.com>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Commit-Queue: Tatsuyuki Ishi <ishitatsuyuki@google.com>

media/ffmpeg/src/error.rs

@@ -27,7 +27,7 @@ mod test {
             ffi::av_strerror(
                 averror,
                 buffer.as_mut_ptr() as *mut c_char,
-                buffer.len() as ffi::size_t,
+                buffer.len() as usize,
             )
         };
         assert_eq!(ret, 0);