Missing EIGEN_DEVICE_FUNCs to get `gpu_basic` passing with CUDA 9.

CUDA 9 seems to require labelling defaulted constructors as EIGEN_DEVICE_FUNC, despite giving warnings that such labels are ignored. Without these labels, the gpu_basic test fails to compile, with errors about calling __host__ functions from __host__ __device__ functions.

With this and !616 (merged), the gpu_basic test now passes for CUDA 9.1.