Include the "stdcpp-path" in the json file

Summary: qmake also generates it and androiddeployqt consumes it.

Test Plan: built and ran kalgebra

Reviewers: Frameworks, vkrause

Reviewed By: vkrause

Subscribers: vkrause, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D11342