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.
Edited by Caio Oliveira