Propagate jsoncpp include path to depenent targets.

This is required in order to land:
https://webrtc-review.googlesource.com/c/src/+/34500.

TBR=phoglund@webrtc.org

Bug: webrtc:8605
Change-Id: Ic5c59b43d7379f0a623b781e55881f8eb2b0075b
Reviewed-on: https://webrtc-review.googlesource.com/44381
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21787}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index ea173ea..f18c0f5 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -496,6 +496,7 @@
     "json.cc",
     "json.h",
   ]
+  all_dependent_configs = [ "//third_party/jsoncpp:jsoncpp_config" ]
   if (rtc_build_json) {
     public_deps = [
       "//third_party/jsoncpp",