[ios] Re-enable warning in //third_party/jsoncpp

The public config //third_party/jsoncpp:jsoncpp_config was
disabling the `deprecated-declarations` warning in all of
its dependencies (which hide other deprecation).

All dependencies on iOS have been fixed, so re-enabled the
warning there. It is not re-enabled on other platforms as
some dependencies cause the warning to fire (mostly in
blink and on Linux).

Bug: 983223, 1258584
Change-Id: Idf2a31780b764fba7624c6f17adca60951b71c9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3424283
Reviewed-by: Jordan Bayles <[email protected]>
Commit-Queue: Jordan Bayles <[email protected]>
Cr-Commit-Position: refs/heads/main@{#966353}
NOKEYCHECK=True
GitOrigin-RevId: 9d36eeb9f89bb800dd4b04d613bc2bf2e942597e
diff --git a/BUILD.gn b/BUILD.gn
index b51aa3a..03d76df 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -11,7 +11,7 @@
   # declaration flag.
   # This temporary flag allowing clients to update to the new version, and then
   # update to the new StreamWriter and CharReader classes.
-  if (!is_win || is_clang) {
+  if (!is_win || (is_clang && !is_ios)) {
     cflags_cc = [ "-Wno-deprecated-declarations" ]
   }
 }