dmesg: make time stamps to be printed consistantly

Earlier uptime determination, which was done with sysinfo(2), had one
second resolution, which made time stamps to be rounded unstable way
depending on when a dmesg command was executed.  In practical terms; the
command below was supposed not to differ but it did.

$ diff -q <(dmesg --ctime) <(sleep 0.5 ; dmesg --ctime)

CC: Kay Sievers <kay@vrfy.org>
Buglink: https://github.com/karelzak/util-linux/issues/24
References: https://lkml.org/lkml/2013/6/30/37
Signed-off-by: Sami Kerola <kerolasa@iki.fi>