libipt: initialize the decoder's time

pt_decoder_init() forgot to call pt_time_init().  This did not have any
negative impact since the decoder was always reset before it was used and
pt_time_init() was called from there.

Nevertheless, fix it.

Change-Id: Icd6dadd952b0c3463f29a6854a128f68382afbae
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>