fix(unix): fix feature gate on WebviewExtUnix

[rhysd]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

Checklist

• This PR will resolve #___
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary
• It can be built on all targets and pass CI/CD.

Other information

WebviewExtUnix trait is only enabled when target_os is "linux". However, as its name says, it should be enabled on all *nix platforms. webkit2gtk crate is also imported with the same feature gate.

I didn't make a change file for this since WebviewExtUnix is not published yet. This change can be amended to changes of #1041.