Make constants/ pass gn check.

Change-Id: I1104d5c3c63dbf9f2f286797db842d30c75e2718
Reviewed-on: https://pdfium-review.googlesource.com/c/44077
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/.gn b/.gn
index 1327f27..d18f4fd 100644
--- a/.gn
+++ b/.gn
@@ -20,6 +20,7 @@
   ":pdfium",
   ":pdfium_embeddertests",
   ":pdfium_unittests",
+  "//constants/*",
   "//core/*",
   "//fpdfsdk/*",
   "//fxbarcode/*",