libudev: Fix redefinition of usec_t and nsec_t.

The type definitions usec_t and nsec_t where defined twice: in
path-util.h and util.h.

time-util.h and time-util.c now gather the time-related functions and
definitions (as in upstream systemd).

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>