Require ostruct to fix test failure

This resolves the following OpenStruct error. There is only 1 reference to OpenStruct in this file, no other references in the code base.

```
NameError:
uninitialized constant RSpec::Rails::OpenStruct
# ./spec/rspec/rails/example/view_example_group_spec.rb:190:in `controller'
# ./spec/rspec/rails/example/view_example_group_spec.rb:196:in `block (3 levels) in <module:Rails>'
# /home/runner/work/rspec-rails/rspec-core/lib/rspec/core/example.rb:263:in `instance_exec'
```

This could be caused by a recent gem update like `json` as I encountered this with serpapi/turbo_tests#49 as I use turbo_tests to run my rspec test suite.

spec/rspec/rails/example/view_example_group_spec.rb

@@ -1,3 +1,4 @@
+require 'ostruct'
 require 'support/group_failure_formatter'
 
 module RSpec::Rails