Doc: Add Q_DECLARE_SHARED_NOT_MOVABLE_UNTIL_QT6 to ignored directives

QDoc needs to ignore Q_DECLARE_SHARED_NOT_MOVABLE_UNTIL_QT6,
introduced in commit 075769ca

, to avoid failures when parsing
the header files.

Change-Id: I07510248590e290e7a2460d51c8333c135647bcb
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>