[zircon] Update cpp-to-rust-coder FFI section to declare functions

GCC requires declarations ahead of definitions for these.

Change-Id: I1329ccb2362f7e655ed1f865aed2ffa3f137ce61
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1706694
Reviewed-by: Adam Barth <abarth@google.com>
Commit-Queue: James Robinson <jamesr@google.com>
Fuchsia-Auto-Submit: James Robinson <jamesr@google.com>