Windows: use GetSystemTimePreciseAsFileTime if available

PostgreSQL on Windows 8 or Windows Server 2012 will now
get high-resolution timestamps by dynamically loading the
GetSystemTimePreciseAsFileTime function. It'll fall back to
to GetSystemTimeAsFileTime if the higher precision variant
isn't found, so the same binaries without problems on older
Windows releases.

No attempt is made to detect the Windows version.  Only the
presence or absence of the desired function is considered.

Craig Ringer