Fix SmartOS compilation error

Refs: https://chromium-review.googlesource.com/c/v8/v8/+/2509594
Change-Id: I98f55b404c0b2b26fbee0453b976119b39255b31
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2565273Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#71681}

src/base/platform/platform-solaris.cc

@@ -66,7 +66,7 @@ void OS::SignalCodeMovingGC() {}
 void OS::AdjustSchedulingParams() {}
 
 // static
-void* Stack::GetStackStart() {
+Stack::StackSlot Stack::GetStackStart() {
   pthread_attr_t attr;
   int error;
   pthread_attr_init(&attr);