cppgc: Fix gcc compilation error

After https://crrev.com/c/3182223 gcc might throw the following
error during compilation:
```
error: variable 'is_on_heap' set but not used
```

Bug: v8:11749
Change-Id: I31a2bef4adb1bfcb2b35115b4dea6df80f84f681
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3183165Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#77067}

src/heap/cppgc/pointer-policies.cc

@@ -86,6 +86,8 @@ void EnabledCheckingPolicyBase::CheckPointerImpl(
       USE(slot_is_live);
     }
   }
+#else
+  USE(is_on_heap);
 #endif  // CPPGC_CHECK_ASSIGNMENTS_IN_PREFINALIZERS
 }