Update scene.h

use #include <cstdlib> only if the compiler is used to compile c++ code.

include/assimp/scene.h

@@ -56,9 +56,9 @@ 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
+#  include <cstdlib>
 extern "C" {
 #endif