Only export symbols from the libraries that should be exported
Summary:
Using generate_export_header instead manually written export headers
should improve coverage across platforms.
Size of stripped libmarblewidget-qt5 reduced on Linux
e.g. from 11077592 to 7889848
Reviewers: rahn, Marble, nienhueser
Reviewed By: Marble, nienhueser
Differential Revision: https://phabricator.kde.org/D2309