New project "public-keys" was added to manifest by [1]. Add it to
.gitignore file.
[1] https://review.opendev.org/c/starlingx/manifest/+/873166
Story: 2009221
Task: 47097
Signed-off-by: Davlet Panech <davlet.panech@windriver.com>
Change-Id: Iafe62c7060d0d3bd538be30e8afe52aeeee3ef44