Consider to fix -Wcast-qual warning in 3 files

[haonanya]

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.