fix: remove mut self from set_native_icon signature

[logankeenan]

What kind of change does this PR introduce?

I'm getting a compile error in the latest alpha release. The error can be reproduced with this repo by running cargo tauri dev. It looks like the bug was introduced here.

error[E0596]: cannot borrow `self_.inner` as mutable, as `self_` is not declared as mutable
   --> /Users/logankeenan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tauri-2.0.0-alpha.14/src/menu/icon.rs:210:49
    |
210 |       return run_main_thread!(self, |self_: Self| self_
    |  _________________________________________________^
211 | |       .inner
212 | |       .set_native_icon(_icon.map(Into::into)));
    | |_____________________________________________^ cannot borrow as mutable
    |
help: consider changing this to be mutable
    |
210 |     return run_main_thread!(self, |mut self_: Self| self_
    |                                    +++

For more information about this error, try `rustc --explain E0596`.
error: could not compile `tauri` (lib) due to previous error

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• 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

Other information

[amrbashir]

Thank you for your contributions! Could you also add a changefile in .changes directory?

[logankeenan]

@amrbashir, thanks for the feedback. All changes have been made