Add missing config to core/fxcrt:seekablemultistream target.

Should have been part of commit e421d384.

TBR=tsepez@chromium.org

Change-Id: I042c1c85f915798dd393f5ea136edb86d644ca11
Reviewed-on: https://pdfium-review.googlesource.com/c/43982
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fxcrt/BUILD.gn

diff --git a/core/fxcrt/BUILD.gn b/core/fxcrt/BUILD.gn
index 1fd82f0..170d4d0 100644
--- a/core/fxcrt/BUILD.gn
+++ b/core/fxcrt/BUILD.gn
@@ -135,6 +135,7 @@
       ":fxcrt",
       "../fpdfapi/parser",
     ]
+    configs += [ "../../:pdfium_core_config" ]
     visibility = [ "../../*" ]
   }
 }