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: NJack Nagel <jacknagel@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

perl/.gitignore

 perl.mak
 perl.mak.old
+MYMETA.json
 MYMETA.yml
 blib
 blibdirs