add -Wcast-qual to CI builds

[bashbaug]

We should add -Wcast-qual to our CI builds to catch casting problems in the future.

Originally posted by @haonanya in #241 (comment):

Which compiler / compiler flags did you use to find these issues? Is this something we could have caught via CI?

gcc uses -Werror -Wcast-qual build fail:

[haonanya]$ /usr/bin/gcc -DCL_ENABLE_LAYERS -DCL_NO_NON_ICD_DISPATCH_EXTENSION_PROTOTYPES -DCL_TARGET_OPENCL_VERSION=300 -DOPENCL_ICD_LOADER_VERSION_MAJOR=3 -DOPENCL_ICD_LOADER_VERSION_MINOR=0 -DOPENCL_ICD_LOADER_VERSION_REV=6 -DOpenCL_EXPORTS -I/export/users/haonanya/OpenCL-CTS/OpenCL-Headers -I/export/users/haonanya/OpenCL-CTS/OpenCL-ICD-Loader/build -I/export/users/haonanya/OpenCL-CTS/OpenCL-ICD-Loader/loader -fPIC -std=gnu99 -Werror -Wcast-qual -MD -MT CMakeFiles/OpenCL.dir/loader/linux/icd_linux.c.o -MF CMakeFiles/OpenCL.dir/loader/linux/icd_linux.c.o.d -o CMakeFiles/OpenCL.dir/loader/linux/icd_linux.c.o -c /export/users/haonanya/OpenCL-CTS/OpenCL-ICD-Loader/loader/linux/icd_linux.c
/export/users/haonanya/OpenCL-CTS/OpenCL-ICD-Loader/loader/linux/icd_linux.c: In function ‘compareDirElem’:
/export/users/haonanya/OpenCL-CTS/OpenCL-ICD-Loader/loader/linux/icd_linux.c:132:21: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
132 | return strcoll(((struct dirElem *)a)->d_name, ((struct dirElem *)b)->d_name);
| ^
/export/users/haonanya/OpenCL-CTS/OpenCL-ICD-Loader/loader/linux/icd_linux.c:132:52: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
132 | return strcoll(((struct dirElem *)a)->d_name, ((struct dirElem *)b)->d_name);
| ^
cc1: all warnings being treated as errors