Check if the application androiddeployqt package has a main() symbol

Summary:
Back in the KDE Edu sprint, we decided we need such a check, otherwise
you get rather frustrated when the application isn't started. A patch to
androiddeployqt was submitted but rejected by the maintainer.
https://codereview.qt-project.org/#/c/207941/

Test Plan: kate doesn't build if we don't pass Q_DECL_EXPORT, builds if we do.

Reviewers: Frameworks, Build System, vkrause

Reviewed By: vkrause

Subscribers: vkrause, vatra, aacid

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D12120