The use of clock_gettime() is limited to systems having _POSIX_TIMERS
defined, however OpenBSD implements clock_gettime() but does not have the
posix define. Enable using clock_gettime() on OpenBSD as well.

Change-Id: I785954fe61b42b15755ca625a766c9a95179ae8d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>