Fix: offset_s and CTF clocks with frequency != 1GHz

Take into account offset_s from the clock description when calculating
the tc_offset in ctf_get_real_timestamp().

Do not assume that the single_clock->offset field is at a frequency of
1GHz. Take into account the its clock frequency.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>