5ttedit: Fix compilation on clang
5ttedit: Fix compilation on clang

Fixes compilation after commit d5c51d15.
Should solve problem reported in #83.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3