Preparation for hidden visibility on gcc

[mvieth]

In voxel_grid_label.cpp, the implementation of VoxelGrid (from voxel_grid.hpp) must be hidden, otherwise VoxelGrid is instantiated again, but with the wrong visibility attributes (would overwrite the correct instantiation from voxel_grid.cpp). Same thing in sac_model_normal_plane.hpp: the implementation of SampleConsensusModelPlane must be hidden. The implementations of dist8 and dist4 are moved from sac_model_plane.hpp to sac_model_plane.h so that they are available when SampleConsensusModelNormalPlane is instantiated and the compiler can optimize.

Split off from https://github.com/PointCloudLibrary/pcl/pull/5779