Init libgit2 submodule if missing

When I use the git repository in a bundler Gemfile like this:

    gem 'rugged', git: 'git://github.com/libgit2/rugged.git', branch: 'development'

It fails because it can't find `Makefile.embed`.

I made `extconf.rb` initialize the libgit2 submodule in this case.