Moves a forward declaration of v8::CFunction to the appropriate
position.

Change-Id: I9ea8c7697a0abd39a0c8fdc536aae0417e04cafe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2359091Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69424}