Update return typehint for SchemaBuilder

[kix]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	nothing to document
Fixed tickets	none
License	MIT

Previously, running bin/console lint:container with this bundle installed lead to this error output:

  Invalid definition for service "overblog_graphql.request_executor": argument 2 of "Overblog\GraphQLBundle\Request\Executor::addSchemaBuilder" accepts "callable", "Closure" passed.

This fixes this error by passing \Closure from \Overblog\GraphQLBundle\Definition\Builder\SchemaBuilder::getBuilder to Executor::addSchemaBuilder.

[mcg-web]

Thank you @kix !