Cuda driver api function cuGetErrorString is available in version 6.0 and
higher.

This patch:
- removes a comment saying the declaration is not available in cuda.h 6.0
- fixes the presence test to use CUDA_VERSION < 6000
- moves the declaration to toplevel

Build on x86_64 with nvptx accelerator and reg-tested libgomp.

2018-08-08  Tom de Vries  <tdevries@suse.de>

	* plugin/plugin-nvptx.c (cuda_error): Move declaration of cuGetErrorString ...
	(cuGetErrorString): ... here.  Guard with CUDA_VERSION < 6000.

From-SVN: r263405