set export-table link arg

[gui1117]

due to rust-lang/rust#57861

new nightlies doesn't set export-table by default.

Also I modified build-demo.sh but this script doesn't work:

error[E0432]: unresolved import `super::Message`
  --> /home/thiolliere/Developpement/polkadot/parachain/src/wasm_api.rs:20:49
   |
20 | use super::{ValidationParams, ValidationResult, Message};
   |                                                 ^^^^^^^ no `Message` in the root

error: aborting due to previous error

and the PR contains new wasm builds

[parity-cla-bot]

It looks like @thiolliere signed our Contributor License Agreement. 👍

Many thanks,

Parity Technologies CLA Bot

[parity-cla-bot]

It looks like @thiolliere signed our Contributor License Agreement. 👍

Many thanks,

Parity Technologies CLA Bot