Test fix

Author: andrjohns

stan/math/opencl/rev/matrix_power.hpp

@@ -34,7 +34,7 @@ inline var_value<matrix_cl<double>> matrix_power(const var_value<T>& M,
   if (M.size() == 0)
     return M;
 
-  size_t N = M.rows();
+  auto N = M.rows();
 
   if (n == 0) {
     return diag_matrix(constant(1.0, N, 1));