Solving a linker error with median_filter.cpp

Solving a linker error with point type XYZI which is not pre-compiled for this filter. The solution that worked was to include the #include <pcl/filters/impl/median_filter.hpp> A better solution is to change the source file to instantiate common point types.
Merge pull request #773 from sidahuja/patch-1

Solving a linker error with median_filter.cpp