Make stylelint ignore the build/ directory

CQ has started failing because now the `build/` dir seems to contain CSS
files that it's linting
(https://ci.chromium.org/ui/p/devtools-frontend/builders/try/devtools_frontend_linux_rel/b8834935955252908353/overview).

Bug: None
Change-Id: I310159cfbb79ed7653ea808d1f67fc3bf0683729
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3190270
Commit-Queue: Jack Franklin <[email protected]>
Commit-Queue: Wolfgang Beyer <[email protected]>
Auto-Submit: Jack Franklin <[email protected]>
Reviewed-by: Wolfgang Beyer <[email protected]>

Author: jackfranklin

.stylelintignore

@@ -8,3 +8,4 @@ test/**/*.css
 test/**/*.ts
 third_party
 front_end/ui/components/docs
+build