Fix error when building assimp on older Mac OS X version.

Prevents this error when building with Mac OS X 10.9 SDK:

    error: no member named 'atoi' in namespace 'std'; did you mean simply 'atoi'?

thirdparty/assimp/include/assimp/scene.h

@@ -56,6 +56,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "material.h"
 #include "anim.h"
 #include "metadata.h"
+#include <cstdlib>
 
 #ifdef __cplusplus
 extern "C" {