app_host_source_dir wrong path

podspec path : /Users/aaa/bbb/xxx/xxx.podspec
yml path : /Users/aaa/bbb/xxx/.gen_config.yml
xcodeproj path : /Users/aaa/bbb/xxx/gen/xxx/xxx.xcodeproj

after set `app_host_source_dir: app` in `.gen_config.yml`

and 'pod gen'

I got an error.

```
Build input file cannot be found: '/Users/aaa/bbb/app/main.m'
```
'/Users/aaa/bbb/**xxx**/app/main.m' expected