add missing lines of code in bspline_fitting tutorial

In the source file in http://pointclouds.org/documentation/tutorials/bspline_fitting.php not the whole code is shown. 
Thus, people are having some trouble http://stackoverflow.com/questions/22293418/error-on-running-bsplines-example-in-pcl-1-7/22890743#22890743 and http://www.pcl-users.org/Fitting-trimmed-B-splines-to-unordered-point-clouds-problem-to-run-td4032826.html

This should solve it.