[1.13.x] require ostruct which was previously required by rake

[rmosolgo]

Fixes #4900

Not needed in 2.x because the file using it isn't present.

There is another use of OpenStruct in the gem, but it will only run in applications which already use OpenStruct:

graphql-ruby/lib/graphql/subscriptions/serialize.rb

Lines 85 to 87 in ec1ae00

	when OPEN_STRUCT_KEY
	ostruct_values = load_value(value[OPEN_STRUCT_KEY])
	OpenStruct.new(ostruct_values)

[rmosolgo]

It looks like a lot of these builds have been failing for a while. I don't plan to dive into that yet ... but it fixed the bug in loading the gem, and the main test suite still passes 🍻