win32: remove non-standard empty struct

As far as I can tell, this is not allowed by the ISO C specification
(it is permitted in C++ though).

src/win32/thread.c

@@ -669,18 +669,18 @@ mtime_t mdate (void)
 
 static union
 {
+#if (_WIN32_WINNT < 0x0601)
     struct
     {
-#if (_WIN32_WINNT < 0x0601)
         BOOL (*query) (PULONGLONG);
-#endif
     } interrupt;
+#endif
+#if (_WIN32_WINNT < 0x0600)
     struct
     {
-#if (_WIN32_WINNT < 0x0600)
         ULONGLONG (*get) (void);
-#endif
     } tick;
+#endif
     struct
     {
         LARGE_INTEGER freq;