Gitignore for Ninja build system dependency-cache and log

Added .gitignore for the [Ninja build system](http://martine.github.io/ninja/). `.ninja_dep` is a cache of dependencies; `.ninja_log` is a build log. These entries are used by, e.g., the ninja build system's .gitignore: https://github.com/martine/ninja/blob/master/.gitignore