Change gitiles:gitiles target visibility to public

This makes it possible to include the plugin as a git submodule
in the plugins folder and make it a core plugin.  Without this
change, the 'release' build fails with:

BUILD FAILED: Couldn't get dependency '//plugins/gitiles:gitiles' of target '//plugins:core':
//plugins:core depends on //plugins/gitiles:gitiles, which is not visible

Change-Id: I154e1cb8ec7b3f1555830f97357526fb372902dd