Add missing definition for val_to_char()

Since the function is inlined the single definition let GCC build everything
properly, but debug builds (which disable inline) resulted in:
slurmstepd: [465.0]: symbol lookup error:
(trimmed path)/task_cgroup.so: undefined symbol: val_to_char
when running srun --cpu_bind=v.

task/affinity had this definition already, task/cgroup didn't.