Audio: Fix Playable.h not included in PlayableGroup.h.

When PlayableGroup.h is used without Playable.h, the compiler spits out a
very unintuitive error message, which would probably confuse unfamiliar
users.
This change fixes that error message appearing.

Signed-off-by: Squareys <Squareys@googlemail.com>

src/Magnum/Audio/PlayableGroup.h

@@ -39,6 +39,7 @@
 #include <Magnum/SceneGraph/FeatureGroup.h>
 
 #include "Magnum/Audio/Audio.h"
+#include "Magnum/Audio/Playable.h"
 #include "Magnum/Audio/Source.h"
 #include "Magnum/Audio/visibility.h"