hddtemp*: avoid using the degrees symbol in vlabel.

The encodings between the different files don't match, and besides, it
seems like even on a fully configured UTF-8 system, an UTF-8 degrees
symbol can throw rrd off.

Every other temperature graph out there uses a variant of 'degrees
Celsius', and that's what the new freeipmi and hwmon plugins use as
well, so that looks like the standard.