run-command.c: make "struct parallel_processes" const if possible

run-command.c: make "struct parallel_processes" const if possible

Add a "const" to two "struct parallel_processes" parameters where
we're not modifying anything in "pp". For kill_children() we'll call
it from both the signal handler, and from run_processes_parallel()
itself. Adding a "const" there makes it clear that we don't need to
modify any state when killing our children.

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