package/qt5/qt5scxml: fix qt5declarative dependency

This commit makes additional improvements after commit
187d2470 ("package/qt5/qt5scxml: needs
qt5declarative package") :

- add qt5declarative depends comment
- add missing qt5declarative dependency

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>