collectd: sensors plugin - depend on lm-sensors

collectd: sensors plugin - depend on lm-sensors

Change the sensors plugin's dependency from libsensors
to lm-sensors, so that also the /usr/sbin/sensors executable
gets installed if the plugin is selected.

Although that is not needed for collectd itself,
the LuCI statistics app expects to find the sensors executable
if the plugin has been installed.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>