Closes https://github.com/assimp/assimp/issues/940: use standard offsetof

instead of an own version.

test/unit/utMaterialSystem.cpp

@@ -43,19 +43,16 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <assimp/scene.h>
 #include <MaterialSystem.h>
 
-
-using namespace std;
-using namespace Assimp;
+using namespace ::std;
+using namespace ::Assimp;
 
 class MaterialSystemTest : public ::testing::Test
 {
 public:
-
     virtual void SetUp() { this->pcMat = new aiMaterial(); }
     virtual void TearDown() { delete this->pcMat; }
 
 protected:
-
     aiMaterial* pcMat;
 };