[asio] add target guard for target asio::asio

[lesomnus]

Describe the pull request

• What does your PR fix?

  Fixes [asio] Prevent add_library(asio::asio) if target already exists. #28298

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

  all, No

• Does your PR follow the maintainer guide?

  Yes

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

  I am still working on this PR

If you are still working on the PR, open it as a Draft: https://github.blog/2019-02-14-introducing-draft-pull-requests/

[lesomnus]

Is it OK to push x-add-version commit? or should I have to wait for someone's approval?

[github-actions]

This is a new experimental fast check for PR issues. Please let us know if this bot is helpful!

PRs must add only one version and must not modify any published versions

When making any changes to a library, the version or port-version in vcpkg.json or CONTROL must be modified.

error: checked-in files for asio have changed but the version was not updated
version: 1.24.0
old SHA: bba8740d419878e427c71f076d569f8a26833c6b
new SHA: f4ac597317008cac0b781ffa1ce1a09e4bc3af87
Did you remember to update the version or port version?
Use --overwrite-version to bypass this check
***No files were updated***

[JonLiu1993]

@lesomnus, Thanks for your pr, please add "port version" : 1, to the vcpkg.json and run command "./vcpkg x-add-version asio --overwrite-version" and commit again.

[lesomnus]

@microsoft-github-policy-service agree