[SDK] driverspecs.h: Add missing dummy _IRQL_limited_to_() definition (when _PREFAST_ is not defined).

Addendum to commit 8a688204.

sdk/include/psdk/driverspecs.h

···
 #define _IRQL_saves_
 #define _IRQL_saves_global_(kind,param)
 #define _IRQL_uses_cancel_
+#define _IRQL_limited_to_(level)
 #define _Kernel_clear_do_init_(yesNo)
 #define _Kernel_float_restored_
 #define _Kernel_float_saved_