x86_64 references a named union, ensure unions are not nameless.

diff --git a/dlls/ntdll/thread.c b/dlls/ntdll/thread.c
index fff3407..cb07fe9 100644
--- a/dlls/ntdll/thread.c
+++ b/dlls/ntdll/thread.c
@@ -29,6 +29,7 @@
 #include <sys/times.h>
 #endif
 
+#define NONAMELESSUNION
 #include "ntstatus.h"
 #include "thread.h"
 #include "winternl.h"