Add missing absl dependency from build.
diff --git a/BUILD.bazel b/BUILD.bazel
index ac62251..b1e3b7f 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -132,6 +132,7 @@
     }),
     deps = select({
         ":has_absl": [
+            "@com_google_absl//absl/container:flat_hash_set",
             "@com_google_absl//absl/debugging:failure_signal_handler",
             "@com_google_absl//absl/debugging:stacktrace",
             "@com_google_absl//absl/debugging:symbolize",