Make ":pdfium" pass gn check.

Change-Id: I41861ae949f44475a13f67e7cb8fb6e186cd6732
Reviewed-on: https://pdfium-review.googlesource.com/c/43990
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/.gn b/.gn
index 8dc9f60..818bf58 100644
--- a/.gn
+++ b/.gn
@@ -17,6 +17,7 @@
 }
 
 check_targets = [
+  ":pdfium",
   "//core/fxcrt/*",
   "//fpdfsdk/*",
   "//fxbarcode/*",