Add the 'a: desktop' label to labeler.yml (#130933)

The desktop team uses the [`a: desktop`](https://github.com/flutter/engine/pulls?q=is%3Aopen+label%3A%22affects%3A+desktop%22+sort%3Aupdated-desc) label to [triage](https://github.com/flutter/flutter/wiki/Triage#desktop-platforms-team-team-desktop) pull requests.

.github/labeler.yml

@@ -13,6 +13,11 @@
   - '**/animation/*'
   - '**/*animation*'
 
+'a: desktop':
+  - '**/linux/**/*'
+  - '**/macos/**/*'
+  - '**/windows/**/*'
+
 'a: internationalization':
   - packages/flutter_localizations/**/*