collectd: Update to 5.4.1

-update collectd to the current release 5.4.1
-fix sigrok glib check (upstream patch after 5.4.1)

Note:
Changes are required also to luci-statistics in Luci source.
Patch in http://patchwork.openwrt.org/patch/5303/

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Merge pull request #298 from hnyman/collectd541

collectd: Update to 5.4.1