src: api.cpp: return 0 for CL_KERNEL_PRIVATE_MEM_SIZE (#327)
src: api.cpp: return 0 for CL_KERNEL_PRIVATE_MEM_SIZE (#327)

CL_KERNEL_PRIVATE_MEM_SIZE is a lower bound of the private memory size
needed by a kernel. Let's return zero not to block application relying
on being able to read this information.
kpet/clvk,kpet/clvk,kpet/clvk,kpet/clvk