gitignore: Ignore repo tool directory

This reduces the noise produced by 'git status' and 'repo status' when
orchestrating the layers with Google's repo tool
(https://gerrit.googlesource.com/git-repo/

).

(From OE-Core rev: 0c2c8d9edcba148c5beb22a9fb8ff06e1ada30cd)

Signed-off-by: Christian Meusel <christian.meusel@posteo.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>