Fixes compile issue for 3ds exporter

... when you have disabled the 3ds importer

code/3DSExporter.cpp

@@ -44,6 +44,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "3DSExporter.h"
 #include "3DSLoader.h"
+#include "3DSHelper.h"
 #include "SceneCombiner.h"
 #include "SplitLargeMeshes.h"
 #include "StringComparison.h"
@@ -54,6 +55,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 using namespace Assimp;
 namespace Assimp    {
+using namespace D3DS;
 
 namespace {