swig-py: Fix a compiler warning about const-ness.

* subversion/bindings/swig/python/libsvn_swig_py/swigutil_py.c
  (make_string_from_ob): Remove the const from the return type.
    No reason to have this here, we're making a copy of the
    string as given by Python.  So we don't need the caller to
    promise to not modify it.  The alternative to fix this would
    be to just cast everywhere which is just ugly.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1597901 13f79535-47bb-0310-9956-ffa450edef68