common: avoid redefining clock type on Windows

common: avoid redefining clock type on Windows

mingw >= 8.0.1 defines CLOCK_REALTIME_COARSE, so we'll avoid
overriding it if already set.

Clock precision [1]:

mingw < 8.0.1:
  * CLOCK_REALTIME: ~10-55ms (GetSystemTimeAsFileTime)
mingw >= 8.0.1:
  * CLOCK_REALTIME: <1us (GetSystemTimePreciseAsFileTime)
  * CLOCK_REALTIME_COARSE: ~10-55ms (GetSystemTimeAsFileTime)

* CLOCK_MONOTONIC: <1us if TSC is usable, ~10-55ms otherwise
                   (QueryPerformanceCounter)

[1] https://github.com/mirror/mingw-w64/commit/dcd990ed423381cf35702df9495d44f1979ebe50

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>