Update median_filter.cpp

Solving a linker error with Point type XYZI which is not precompiled for this filter. The solution that worked was to add #include <pcl/filters/impl/median_filter.hpp>. The correct way to make it more generic is to use :
PCL_INSTANTIATE(MedianFilter, PCL_XYZ_POINT_TYPES) 
in the .cpp source file so that this instantiation is triggered in the corresponding .cpp file.