pallet-contracts: Fix compilation on latest nightly (#2986)

Apparently they changed detection for visibility identifiers on traits,
which broke more than it should. There is an issue open:
https://github.com/rust-lang/rust/issues/119924 The easy solution for us
is to move the declaration of the global variable outside of the trait.

Closes: https://github.com/paritytech/polkadot-sdk/issues/2960