Fix compilation on Windows

Summary:
Two issues preventing compilation on windows :

• ResourceInterface was missing an export definition
• The gstreamer/qtmultimedia backends plugins were using the same export include than libsound

Reviewers: Artikulate, KDE Edu, pino, cordlandwehr

Reviewed By: Artikulate, cordlandwehr

Subscribers: cordlandwehr, pino, kde-edu

Tags: KDE Edu, Windows

Differential Revision: https://phabricator.kde.org/D15914