Add missing config to new image_diff target.

This fixes building PDFium as part of another project like Chromium.

Change-Id: Ide4d04072135345c0b2d2cfe6a1ab2246f5d3418
Reviewed-on: https://pdfium-review.googlesource.com/6970
Commit-Queue: Lei Zhang <thestig@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index f577986..5b64e04 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -266,6 +266,7 @@
     "testing/image_diff/image_diff_png.cpp",
     "testing/image_diff/image_diff_png.h",
   ]
+  configs += [ ":pdfium_core_config" ]
   deps = []
   if (!pdf_enable_xfa) {
     deps += [ "third_party:fx_lpng" ]