Improve port of struct timespec to MS-Windows.

Improve port of struct timespec to MS-Windows.

 lib/makefile.w32-in ($(BLD)/dtotimespec.$(O)):
 ($(BLD)/timespec-add.$(O)):
 ($(BLD)/timespec-sub.$(O)): Don't depend on
 $(EMACS_ROOT)/nt/inc/sys/time.h.
 lib/stat-time.h:
 lib/timespec.h:
 lib/utimens.h: Revert last change.
 src/makefile.w32-in (TIMESPEC_H): Remove nt/inc/sys/time.h.
 (SYSTIME_H): Add nt/inc/sys/time.h.
 src/systime.h [WINDOWSNT]: Include sys/time.h.
 src/s/ms-w32.h (struct timespec): Definition moved from
 nt/inc/sys/time.h.  Suggested by Paul Eggert <eggert@cs.ucla.edu>.
 nt/inc/sys/time.h (struct timespec): Don't define it here, it is
 now defined in src/s/ms-w32.h.