package/assimp: fix namespace related compile failure

Fixes:

  - http://autobuild.buildroot.net/results/0861c66ec02a55e984577094e28b65c78b95a330



  .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderScene.cpp:101:33: error: specialization of 'template<class T> void Assimp::Blender::Structure::Convert(T&, const Assimp::Blender::FileDatabase&) const' in different namespace [-fpermissive]
           const FileDatabase &db) const {
                                   ^
 In file included from .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderScene.h:49:0,
                   from .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderScene.cpp:47:
  .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderDNA.h:266:10: error:   from definition of 'template<class T> void Assimp::Blender::Structure::Convert(T&, const Assimp::Blender::FileDatabase&) const' [-fpermissive]
       void Convert(T &dest, const FileDatabase &db) const;
            ^
  .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderScene.cpp:121:33: error: specialization of 'template<class T> void Assimp::Blender::Structure::Convert(T&, const Assimp::Blender::FileDatabase&) const' in different namespace [-fpermissive]
           const FileDatabase &db) const {
                                   ^
  In file included from .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderScene.h:49:0,
                   from .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderScene.cpp:47:
  .../build/assimp-5.1.4/code/AssetLib/Blender/BlenderDNA.h:266:10: error:   from definition of 'template<class T> void Assimp::Blender::Structure::Convert(T&, const Assimp::Blender::FileDatabase&) const' [-fpermissive]
       void Convert(T &dest, const FileDatabase &db) const;
            ^

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[yann.morin.1998@free.fr: update upstream status]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>