Uptime plugin fails to adjust for system time changes after boot.
As Openwrt/LEDE routers usually do not have a RTC, the system time
gets adjusted with NTP possibly after collectd has already started.
But collectd continues to use the initial time set by 'sysfixtime',
which can lead to incorrect uptime calculations.

Apply a proposed fix from upstream that uses /proc/uptime
Reference to https://github.com/collectd/collectd/pull/2034

Fixes #4819

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