[build] Explicitly add -Wno-deprecated-copy

Parts of the FIDL C++ bindings fail to build if deprecated-copy warnings
are enabled and fatal so we should stop setting them by default in the
main fuchsia.git repo. But parts of Tink don't trigger the warning. This
explicitly suppresses that in this repo.

Bug: 77383
Bug: 38640
Change-Id: I0d47f390d6d2f6b4de74437e92ea87b840aa5985
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/tink/+/535901
Fuchsia-Auto-Submit: Ian McKellar <ianloic@google.com>
Reviewed-by: Yifei Teng <yifeit@google.com>
Commit-Queue: Ian McKellar <ianloic@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index a519c18..bf7f4be 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,6 +8,9 @@
 
     # The tink library uses absl headers containing deprecated API usage.
     "-Wno-deprecated-declarations",
+
+    # The tink library uses absl headers with implicit copy constructors.
+    "-Wno-deprecated-copy",
   ]
 
   include_dirs = [