Add MYMETA.json to perl/.gitignore

ExtUtils::MakeMaker generates MYMETA.json in addition to MYMETA.yml since version 6.57_07. As it suggests, it is just meta information about the build and is cleaned up with 'make clean', so it should be ignored. Signed-off-by: Jack Nagel <jacknagel@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>