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 #241 to fix them.