Fix segfault when running with Flutter 3.0 (#663)

The release of Flutter 3.0 caused apps to segfault when running with hover.
The issue seems to be related to an updated ABI that required new headers to be
compiled into `go-flutter` to make the interop compatible.

This commit updates the `embedder.h` file to match the latest file from
`flutter/flutter` allowing applications to run with the new version.

Author: bltavares