Fix distutils 'setup.py' to solve problem with 'make install'.
Fix distutils 'setup.py' to solve problem with 'make install'.

In a previous commit, a line from the 'setup.py' was removed. After this, 'make install' complains that there is no directory named 'viennagrid'. Adding this line again solves the issue.

jonancm/viennagrid-python,jonancm/viennagrid-python,jonancm/viennagrid-python