eudev: fix redefinition of usec_t and nsec_t.

usec_t and nsec_t are defined twice (src/libudev/{util.h,path-util.h}).
This raises an error when using GCC 4.5, but not with later versions!
This patch fixes the issue by gathering the time-related functions and
definitions to time-util.[ch], as in upstream systemd.

This patch has been sent to upstream.

Fixes http://autobuild.buildroot.net/results/060/0605f279abfdfc837f6973f2898ed7ee39f2b8d1/



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>