Run GN check on third_party/icu.

ICU passes GN check now. See crrev.com/919659 for context.

Change-Id: I221285a4fc37a4aabcc0327dfa26644783930c7a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/100832
Auto-Submit: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/.gn b/.gn
index 16cdbfa..59da479 100644
--- a/.gn
+++ b/.gn
@@ -21,7 +21,6 @@
 }
 
 no_check_targets = [
-  "//third_party/icu/*",
   "//v8:cppgc_base",
   "//v8:v8_internal_headers",
   "//v8/src/inspector:inspector",