collectd: uptime plugin: apply fix from upstream

collectd: uptime plugin: apply fix from upstream

Backport from master the fix for uptime plugin.
Adjust it for 5.5.3

  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

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