trace2: use designated initializers for "struct tr2_tgt"

trace2: use designated initializers for "struct tr2_tgt"

As with the preceding commit, change another set of file-level struct
assignments to use designated initializers.

As before the "= NULL" assignments are redundant, but we're keeping
them for self-documentation purposes. The comments left to explain the
pre-image can now be removed in favor of working code that relays the
same information to the reader.

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