[cleanup] Only include halfsiphash.h when libsiphash is used

R=yangguo@chromium.org

Change-Id: If4c158e339b603268563223d900e6879f43ec63b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1725611
Commit-Queue: Simon Zünd <szuend@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62971}

src/utils/utils.h

@@ -20,10 +20,13 @@
 #include "src/base/platform/platform.h"
 #include "src/base/v8-fallthrough.h"
 #include "src/common/globals.h"
-#include "src/third_party/siphash/halfsiphash.h"
 #include "src/utils/allocation.h"
 #include "src/utils/vector.h"
 
+#if defined(V8_USE_SIPHASH)
+#include "src/third_party/siphash/halfsiphash.h"
+#endif
+
 #if defined(V8_OS_AIX)
 #include <fenv.h>  // NOLINT(build/c++11)
 #endif