Revert "package/collectd: disable libupsclient"

This reverts commit 0be1c3e9.

The actual issue is more complex. The problem purportedly fixed was not
caused by a missing libupsclient (it was present), but by a missing type
definition for time_t (on a musl toolchain).

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>