Consider to fix -Wcast-qual warning in 3 files

ocl-icd-src/loader/icd.c:329:23: error: cast from 'const struct _cl_icd_dispatch *' to 'void **' drops const qualifier [-Werror,-Wcast-qual]
329 | ((void **)layerDispatch)[i] ?
ocl-icd-src/loader/linux/icd_linux.c:132:39
ocl-icd-src/loader/icd_dispatch.c::44

opencl-icd-loader fail with -Werror,-Wcast-qual.
tag @bashbaug, @Kerilk

Drafted https://github.com/KhronosGroup/OpenCL-ICD-Loader/pull/241 to fix them.

Start with the compiler diagnostics at the listed lines in ocl-icd-src/loader/icd.c, ocl-icd-src/loader/linux/icd_linux.c, and ocl-icd-src/loader/icd_dispatch.c. Build OpenCL-ICD-Loader with -Werror and -Wcast-qual, then verify all three diagnostics are gone without introducing new warnings.

Written by the indexing model from the issue text.