Fix OSS QUICHE build.

Add a missing dependency in BUILD.bazel.

PiperOrigin-RevId: 919770700

quiche/BUILD.bazel

diff --git a/quiche/BUILD.bazel b/quiche/BUILD.bazel
index 498ea7c..1aef208 100644
--- a/quiche/BUILD.bazel
+++ b/quiche/BUILD.bazel
@@ -583,6 +583,7 @@
         ":quiche_tool_support",
         "@com_google_absl//absl/base",
         "@com_google_absl//absl/base:nullability",
+        "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/functional:bind_front",
         "@com_google_absl//absl/hash",
         "@com_google_absl//absl/memory",