lmsensors-config: fix a warning about not existant S

Set the S variable to $WORKDIR to remove the warning about S being not
existant (which is true for this package, since there are no original
sources for it).

Signed-off-by: Dmitry Eremin-Solenikov <dmitry_eremin@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>