FBX-Importer: fix usage of deprecated include.

code/FBXConverter.cpp

@@ -61,7 +61,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <memory>
 #include <iterator>
 #include <vector>
-#include <strstream>
+#include <sstream>
 #include <iomanip>
 
 namespace Assimp {