Bug#761528: viennacl should requires opencl-dev

[Mathieu Malaterre]

Control: tags -1 patch

I believe viennacl should B-D on opencl-dev instead of libopencl1.

As per build log:

[...]
-- Could NOT find OPENCL (missing:  OPENCL_LIBRARY)
CMake Warning at cmake/ViennaCLCommon.cmake:151 (export):
  Cannot create package registry file:
[...]

The *.so symlink is provided only in the -dev package.

BTW the cmake script is bogus, since compilation should not go any
further, maybe a find_package(OpenCL REQUIRED) is missing. It would
save some compilation time next time.

2cts