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/
```

bindings/ruby/.gitignore

@@ -1,3 +1,8 @@
 LICENSE
 pkg/
 lib/whisper.*
+ext/examples/
+ext/ggml/
+ext/include/
+ext/scripts/
+ext/src/