Declaration-definition consistency nbxn_cuda_wait_gpu.

Changed float->real in nbnxn_cuda_wait_gpu() for consistency between
function declaration and function definition.  Added note that this is
only implemented for single-precision at the moment.  CMake will
complain if GMX_GPU and GMX_DOUBLE are both set.
Addresses Bug #1157.

Change-Id: Ide495cbaba6d120d91f106c6a87ca04e46a2f5a8