sysutils/slurm-wlm: More precise hdf5 dependency

Change LIB_DEPEND to libhdf5_cpp.so, since slurm depends on h5cc, which
is conditionally installed when C++ support is enabled. This is a slight
improvement for users building slurm from source when hdf5 C++ support is
missing.

PR: 253599
Reported by: Patrick McMunn