Mark gitk script executable

The executable bit on gitk-git/gitk was lost (accidentally it seems) by commit 62ba5143ec2ab9d4083669b1b1679355e7639cd5. Put it back, so that gitk can be run directly from a git.git checkout. Note that the script is already executable in gitk.git, just not in git.git. Signed-off-by: Anders Kaseorg <andersk@mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>