unbundle: provide shim headers for system ffmpeg

See https://bugs.gentoo.org/show_bug.cgi?id=576372 for context.

BUG=none
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1894443003 .

Cr-Commit-Position: refs/heads/master@{#387889}
1 file changed