loss_interval: Fix timeval initialisation

When compiling with EXTRA_CFLAGS=-W noticed that tstamp is not initialised
correctly in dccp_li_calc_first_li.

Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>