spirv: Update the OpenCL.std.h header

This corresponds to commit 8b911bd2ba37677037b38c9bd286c7c05701bcda on
GitHub.

We previously tweaked OpenCL.std.h from upstream to be included in C
code.  Now upstream header can be included, however the symbol names
are slightly different (include an OpenCLstd_ prefix), so this patch
also fixes vtn_opencl.c to use those.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
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merge request