trace2: use designated initializers for "struct tr2_dst"

trace2: use designated initializers for "struct tr2_dst"

Convert the "static struct tr2_dst" assignments in trace2/* to use
designated initializers. I don't think it improves readability to
include the explicit 0-ing out of the
fd/initialized/need_close/too_many_files members, so let's have those
be initialized implicitly by the compiler.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>