agent: Fix cargo test -p agent on windows (#52311)

Release Notes:

- N/A or Added/Fixed/Improved ...

crates/gpui_platform/Cargo.toml

@@ -28,6 +28,7 @@ gpui_macos.workspace = true
 
 [target.'cfg(target_os = "windows")'.dependencies]
 gpui_windows.workspace = true
+gpui = { workspace = true, features = ["windows-manifest"] }
 
 [target.'cfg(any(target_os = "linux", target_os = "freebsd"))'.dependencies]
 gpui_linux.workspace = true