qtwinextras: add qtdeclarative as an optional dependency

qtwinextras contains optional QML types, so qtdeclarative must
be listed as an optional dependency to avoid build breaks.

Change-Id: I7d7f3bfca73c6599699a1da9c8795e6ac9ca4a74
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>