[iwyu] Add missing forward declaration

The missing forward declaration made include header checks fail on gcc:
https://crrev.com/c/1637464

R=ishell@chromium.org

Bug: v8:9290,  v8:7490,  v8:9183
Change-Id: I7e513c04297982e403783e7ea7341b271c4fef72
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1640214Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61965}

src/compiler/backend/unwinding-info-writer.h

@@ -23,6 +23,7 @@ namespace v8 {
 namespace internal {
 
 class EhFrameWriter;
+class Zone;
 
 namespace compiler {