[bazel] Adds unmarker.h

No-Try: true
Bug: v8:12324
Change-Id: I4d8ea351b0aecaad704a0d0aa2e67eff4ce5eff0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3226336
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77449}

BUILD.bazel

@@ -2714,6 +2714,7 @@ filegroup(
         "src/heap/cppgc/task-handle.h",
         "src/heap/cppgc/trace-event.h",
         "src/heap/cppgc/trace-trait.cc",
+        "src/heap/cppgc/unmarker.h",
         "src/heap/cppgc/virtual-memory.cc",
         "src/heap/cppgc/virtual-memory.h",
         "src/heap/cppgc/visitor.cc",