Added missing @discardableResult

SwiftyStoreKit/ProductsInfoController.swift

@@ -51,6 +51,7 @@ class ProductsInfoController: NSObject {
     // As we can have multiple inflight requests, we store them in a dictionary by product ids
     private var inflightRequests: [Set<String>: InAppProductQuery] = [:]
 
+    @discardableResult
     func retrieveProductsInfo(_ productIds: Set<String>, completion: @escaping (RetrieveResults) -> Void) -> InAppProductRequest {
 
         if inflightRequests[productIds] == nil {