Reduce database include dependencies

src/database/Playlist.cpp

@@ -16,8 +16,12 @@
  * You should have received a copy of the GNU General Public License
  * along with LMS.  If not, see <http://www.gnu.org/licenses/>.
  */
+#include <cassert>
 
-#include "Types.hpp"
+#include "User.hpp"
+#include "Track.hpp"
+
+#include "Playlist.hpp"
 
 namespace Database {