build: use correct libraries for clock_gettime

build: use correct libraries for clock_gettime

On OpenBSD, clock_gettime() exists in libc rather than librt, and
blindly linking with -lrt made the build fail.  Gnulib already
did the work for determining which libraries to use, so we should
reuse that work rather than doing it ourselves.

* bootstrap.conf (gnulib_modules): Pull in clock-time.
* configure.ac (RT_LIBS): Drop.
* src/Makefile.am (libvirt_util_la_LIBADD): Use gnulib variable
instead.
* src/util/virtime.c (includes): Simplify.