[sdl2] add 'wayland' feature

[quyykk]

• What does your PR fix?

Adds a new feature to the sdl2 port for loading wayland shared.

• Which triplets are supported/not supported? Have you updated the CI baseline?

linux only

• Does your PR follow the maintainer guide?

yeah

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

yeah

[Cheney-W]

All features passed with x64-linux triplet.