windows: Fix a build error.

windows: Fix a build error.

  serialport.c: In function 'get_time':
  serialport.c:64:6: warning: implicit declaration of function 'clock_gettime' [-Wimplicit-function-declaration]
    if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1)
        ^
  serialport.c:64:20: error: 'CLOCK_MONOTONIC' undeclared (first use in this function)
    if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1)
                      ^
  serialport.c:64:20: note: each undeclared identifier is reported only once for each function it appears in
  serialport.c:65:17: error: 'CLOCK_REALTIME' undeclared (first use in this function)
     clock_gettime(CLOCK_REALTIME, &ts);
                   ^
  serialport.c: At top level:
  serialport.c:60:13: warning: 'get_time' defined but not used [-Wunused-function]
   static void get_time(struct timeval *time)
               ^