cppgc: Fix caged-heap.h for the test-include script

This wraps CagedHeap in #if defined(CPPGC_CAGED_HEAP)

Bug: v8:12231
Change-Id: I8407abdd88c6a96b6c79c08e7039b5eb6f8175f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695384Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81010}

include/cppgc/internal/caged-heap.h

@@ -11,6 +11,8 @@
 #include "cppgc/internal/base-page-handle.h"
 #include "v8config.h"  // NOLINT(build/include_directory)
 
+#if defined(CPPGC_CAGED_HEAP)
+
 namespace cppgc {
 namespace internal {
 
@@ -38,4 +40,6 @@ class V8_EXPORT CagedHeapBase {
 }  // namespace internal
 }  // namespace cppgc
 
+#endif  // defined(CPPGC_CAGED_HEAP)
+
 #endif  // INCLUDE_CPPGC_INTERNAL_CAGED_HEAP_H_