Add support for collectd-mod-uptime to luci-statistics

collectd-mod-uptime is already there, but there has been no Luci support for
it, so no graphs are shown.

Based on forum discussion (see
https://forum.openwrt.org/viewtopic.php?id=42478 ), I submit a patch to add
the support to luci-statistics.

Patch is for Luci trunk and the functionality can be enabled by selecting
collectd-mod-uptime in menuconfig.

signed-off-by: hannu.nyman@iki.fi

git-svn-id: http://svn.luci.subsignal.org/luci/trunk@9812 ab181a69-ba2e-0410-a84d-ff88ab4c47bc