cmake/modules/FindVIGRA.cmake: Fix setting VIGRA_ROOT.
cmake/modules/FindVIGRA.cmake: Fix setting VIGRA_ROOT.

nanshe-org/rank_filter,jakirkham/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter,DudLab/rank_filter,jakirkham/rank_filter,jakirkham/rank_filter