dev-util/opencl-headers: install directly into /usr/include

dev-util/opencl-headers: install directly into /usr/include

These cannot be in /usr/lib/OpenCL/vendor/... because eselect-opencl
doesn't know how to handle unified headers, resulting in build failures
due to missing header files. Seeing as we are planning to deprecate
eselect-opencl soon anyway, adapt this and opencl-icd-loader to the new
approach so that they can be used for transition testing without
breaking the more established ICD loader i.e. ocl-icd.

Note that this WILL cause massive dependency issues until
opencl-icd-loader has been added to virtual/opencl. Use with caution.

Signed-off-by: Marek Szuba <marecki@gentoo.org>