stat-time, timespec, u64: support naive out-of-dir builds

stat-time, timespec, u64: support naive out-of-dir builds

* lib/stat-time.c, lib/timespec.c, lib/u64.c:
Use '#include "foo.h"', not '#include <foo.h>', when including
one's own interface.  This works better when configuring with
out-of-directory builds, since packages need not add an
otherwise-unnecessary -I$(topdir_src)/lib to DEFAULT_INCLUDES.