Add third_party/jsoncpp/source to .gitignore

Follow-up to https://chromium-review.googlesource.com/c/v8/v8/+/1947688,
which unhid the third_party/jsoncpp directory, but the source subdir is
still synced via gclient so needs hiding.

Change-Id: Ica6f7e63e92dd9fa5a460e844a7d273f2ebe109c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2169102
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67463}

.gitignore

@@ -63,6 +63,7 @@
 !/third_party/binutils
 !/third_party/inspector_protocol
 !/third_party/jsoncpp
+/third_party/jsoncpp/source
 !/third_party/colorama
 /third_party/colorama/src
 !/third_party/googletest