[turboshaft] fix gcc build for SnapshotTable

https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20gcc%20-%20builder/1452/overview

Change-Id: Ifa02b725a6e01976f2eb5b54d427acf137e6c2f8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3990830
Reviewed-by: Darius Mercadier <dmercadier@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83989}

src/compiler/turboshaft/snapshot-table.h

@@ -28,7 +28,9 @@
 //   NewKey()           O(1)
 namespace v8::internal::compiler::turboshaft {
 
-template <class Value, class KeyData = std::monostate>
+struct NoKeyData {};
+
+template <class Value, class KeyData = NoKeyData>
 class SnapshotTable {
  private:
   struct TableEntry;