media: ffmpeg: Fix AvPacket test not compiling.

Changed missed by accident in 61d72266 (media: ffmpeg: Take AvBuffer
directly in AvPacket::new_owned). The later commits were tested but it
was grouped into the wrong commit in rebase.

BUG=b:239897269
TEST=cargo test --features "video-decoder,ffmpeg" -p ffmpeg -p devices

Change-Id: Iabdcc98833b24271f4701fb210e0e5733d1f9f12
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3868594
Tested-by: Tatsuyuki Ishi <ishitatsuyuki@google.com>
Commit-Queue: Junichi Uekawa <uekawa@chromium.org>
Reviewed-by: Junichi Uekawa <uekawa@chromium.org>

media/ffmpeg/src/avcodec.rs

@@ -658,11 +658,11 @@ mod tests {
 
         let pkt = AvPacket::new_owned(
             0,
-            DropTestBufferSource {
+            AvBuffer::new(DropTestBufferSource {
                 dropped: dropped.clone(),
-            },
-        )
-        .unwrap();
+            })
+            .unwrap(),
+        );
         assert!(!dropped.load(Ordering::SeqCst));
         drop(pkt);
         assert!(dropped.load(Ordering::SeqCst));