Fixed compilation warning

unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
index 620666c..52cfc28 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
@@ -710,7 +710,7 @@
   {
     // Handle degenerate intervals by gracefully clamping and allowing m_dimensions to be zero
     DSizes<Index,NumDims> startIndicesClamped, stopIndicesClamped;
-    for (int i = 0; i < internal::array_size<Dimensions>::value; ++i) {
+    for (size_t i = 0; i < internal::array_size<Dimensions>::value; ++i) {
       eigen_assert(m_strides[i] != 0 && "0 stride is invalid");
       if(m_strides[i]>0){
         startIndicesClamped[i] = clamp(op.startIndices()[i], 0, m_impl.dimensions()[i]);