Add ostruct to Gemfile

ostruct is no longer a default gem in 4.0 so we need to include it in
the gemspec

Output from the test suite without adding the gem:

```
An error occurred while loading ./spec/lib/tasks/data_hygiene/duplicate_report_spec.rb. - Did you mean?
                    rspec ./spec/lib/tasks/data_hygiene/publishing_delay_reporter_spec.rb

Failure/Error: require "ostruct"

LoadError:
  cannot load such file -- ostruct
--

Author: yndajas

Gemfile

@@ -8,6 +8,7 @@ gem "deepsort"
 gem "gds-api-adapters"
 gem "gds-sso"
 gem "govuk_app_config"
+gem "ostruct"
 gem "pg"
 gem "plek"
 gem "rack-cache"

Gemfile.lock

@@ -460,6 +460,7 @@ GEM
       opentelemetry-semantic_conventions
     opentelemetry-semantic_conventions (1.37.0)
       opentelemetry-api (~> 1.0)
+    ostruct (0.6.3)
     pact (1.67.5)
       pact-mock_service (~> 3.0, >= 3.3.1)
       pact-support (~> 1.21, >= 1.21.2)
@@ -722,6 +723,7 @@ DEPENDENCIES
   govuk_test
   listen
   openapi3_parser (~> 0.10.1)
+  ostruct
   pact (~> 1.67)
   pg
   plek