undid accidental change of Shuffle()

Source/Math/CntkBatchNormalization.cuh

@@ -108,8 +108,9 @@ __device__ __forceinline__ void StoreValues<4, float>(const float src[4], float*
 template <typename T>
 __device__ __forceinline__ T Shuffle(T input, int srcLane)
 {
-#if __CUDA_ARCH__ >= 300
+#ifdef __CUDA_ARCH__
     // shfl is supported only on Kepler+
+    static_assert(__CUDA_ARCH__ >= 300, "CNTK only supports only Kepler GPU architecture or newer.");
     return cub::ShuffleIndex(input, srcLane);
 #else
     assert(false);