cuda_wrapper: disable warning for CUDA 5.5 headers

Clang ≥ 3.3 emits a warning for CUDA 5.5 headers:

    /usr/local/cuda-5.5/include/cuda_runtime.h:225:33: warning: function
    'cudaMallocHost' is not needed and will not be emitted
    [-Wunneeded-internal-declaration]
texture: replace private reference by pointer

A default initialisation of a reference is not possible without
potentially dangling objects. (Clang ≥ 3.3 and GCC ≥ 4.8 complain about
this.) The use of pointers is semantically equivalent and safer.