Add missing #include <QQuaternion>.

Qt6 dropped transitive includes that Qt5 used to provide, so the bare
`QQuaternion` reference in PlottableBody's transform() signature now
fails to compile (incomplete type). Adding the explicit include fixes
the build without rippling through call sites.

verified 2026-05-14 against bornagain-23.0 + dev-qt/qtbase:6 6.10.3.

--- a/Img3D/Model/PlottableBody.h
+++ b/Img3D/Model/PlottableBody.h
@@ -19,6 +19,7 @@
 #include "Img3D/Model/Geometry_inc.h"
 #include "Img3D/Type/FloatVector3D.h"
 #include <QColor>
+#include <QQuaternion>
 #include <QMatrix4x4>

 namespace Img3D {