timesync/timesyncd-manager: fix format-specifier issue

timex::time::tv_sec can have different sizes depending on the
host architecture. On x32 in particular, it is 8 bytes
long while the long int type is only 4 bytes  long. Hence,
using li as a format specifier will trigger a format
error. Thus, better use PRI_TIME instead of li which is
actually the right format specifier to use for time_t.