Add a pdfium build config in constants/BUILD.gn.

Otherwise the build will not work outside of a standalone PDFium
checkout, as constants/ build rules do have specify the PDFium source
directory as an include directory. This broke in commit
ef2350308c3212e40fac6a182322b38be42e231d.

Change-Id: Id52840e93d0158e69b753c6adb8211b3ed03f147
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/81670
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

constants/BUILD.gn

diff --git a/constants/BUILD.gn b/constants/BUILD.gn
index 87fc747..1c7cea4 100644
--- a/constants/BUILD.gn
+++ b/constants/BUILD.gn
@@ -22,4 +22,6 @@
     "transparency.cpp",
     "transparency.h",
   ]
+
+  configs += [ "../:pdfium_strict_config" ]
 }