ruby : add .gitignore entries for ext directory (#3245)

ruby : add .gitignore entries for ext directory (#3245)

This commit adds entries to `.gitignore` for directories in the
`ext` directory.

The motivation for this is that currently after building locally these
following files are reported by git as untracked:
```console
Untracked files:
  (use "git add <file>..." to include in what will be committed)
	ext/examples/
	ext/ggml/
	ext/include/
	ext/scripts/
	ext/src/
```