gpui: Fix manifest path for cross-compilation (#62525)

# Objective

Fixes #62522

`cargo xwin` fails to find the GPUI Windows manifest because `llvm-rc`
resolves the path from the resource directory.

## Solution

Reference the manifest relative to `gpui.rc` and explicitly add the
resource directory to the include paths. This keeps both native Windows
and xwin builds working.

## Testing

- Tested with `cargo xwin clippy --target x86_64-pc-windows-msvc`

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- N/A
- [GPUI] N/A

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Co-authored-by: Chris Biscardi <chris@christopherbiscardi.com>
c24e309d93zooeywm committed on 9/17/2026, 5:38:24 PM· committed by GitHubparentcf4deb2
2 files changedLine totals unavailable