src/libudev/util.h: import FORMAT_TIMESPAN_MAX for selinux

When configure --enable-selinux, we hit a compile time error
with a missing definition of FORMAT_TIMESPAN_MAX.  This
was imported from upstream's src/shared/time-util.h into
our src/libudev/util.h.

Thanks Amadeusz Sławiński <amade@asmblr.net>

X-Gentoo-Bug: 499252
X-Gentoo-Bug-URL: https://bugs.gentoo.org/499252

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>