Fixed build warnings on MSVC14 x64 in X file importer.

code/XFileImporter.cpp

@@ -286,7 +286,7 @@ void XFileImporter::CreateMeshes( aiScene* pScene, aiNode* pNode, const std::vec
             // or referenced material, it should already have a valid index
             if( sourceMesh->mFaceMaterials.size() > 0)
             {
-        mesh->mMaterialIndex = sourceMesh->mMaterials[b].sceneIndex;
+                mesh->mMaterialIndex = static_cast<unsigned int>(sourceMesh->mMaterials[b].sceneIndex);
             } else
             {
                 mesh->mMaterialIndex = 0;