Mark assimp as security critical

The assimp scene importer plugin feeds arbitrary 3D asset files to
assimp, including content that QSceneLoader downloads from remote
URLs. assimp therefore processes untrusted content, and is already
marked as security critical in qtquick3d.

Pick-to: 6.12 6.12.0
Change-Id: I0c8e406cd9775b04f10734fdef915193db21cdd8
Reviewed-by: Mike Krus <mike.krus@kdab.com>