[sample_consensus] Fix sign-compare warning in Ellipse3D model

Correct loop index type to Eigen::Index to match signedness of .size()
and satisfy strict CI compiler flags.

Author: AlrIsmail

sample_consensus/src/sac_model_ellipse3d.cpp

@@ -319,7 +319,7 @@ namespace pcl
 
       // Iterate through the 3D points and calculate the distances from them to the ellipse
       distances.resize (x.size ());
-      for (std::size_t i = 0; i < x.size (); ++i)
+      for (Eigen::Index i = 0; i < x.size (); ++i)
       {
         float th_opt;
         const Eigen::Vector3f p (x [i], y [i], z [i]);