Fix build of pdfium_diff when coverage is enabled

Missing config will cause the pdfium_diff build to fail, since it will
not be able to resolve the coverage instrument calls when linking.

Change-Id: I1940432fe7176cad5da80de5e23aad9be188eb69
Reviewed-on: https://pdfium-review.googlesource.com/7870
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 63ba46a..7581187 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2080,6 +2080,7 @@
     ]
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
+    configs += [ ":pdfium_core_config" ]
   }
 
   group("fuzzers") {