timesync/timesyncd-manager: fix format-specifier issues

timex::time::tv_usec and timex::freq 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, introduce a new format specifier
PRI_TIMEX which is defined as PRIi64 on x32 and li
everywhere else.