Fix typo.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/21045002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15924 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/platform-win32.cc

@@ -1359,7 +1359,7 @@ int OS::StackWalk(Vector<OS::StackFrame> frames) {
   // Initialize the stack walking
   STACKFRAME64 stack_frame;
   memset(&stack_frame, 0, sizeof(stack_frame));
-#if V8_OS _WIN64
+#if V8_OS_WIN64
   stack_frame.AddrPC.Offset = context.Rip;
   stack_frame.AddrFrame.Offset = context.Rbp;
   stack_frame.AddrStack.Offset = context.Rsp;