FindHDF5: fix variable name when querying `hdf5_hl`'s implib
Introduced in 4150048a (FindHDF5: unset some variables, 2017-07-24).
Introduced in !1064 (merged).
Backport: release
Introduced in 4150048a (FindHDF5: unset some variables, 2017-07-24).
Introduced in !1064 (merged).
Backport: release