Mark gitk script executable

The executable bit on gitk-git/gitk was lost (accidentally it seems) by
commit 62ba514.  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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: andersk