p0004: make perf test executable

It looks like in 89c3b0ad43 (name-hash: add perf test for lazy_init_name_hash, 2017-03-23) p0004 was not created with the execute unix rights. Let's fix that. Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Acked-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>