Fix warning: incompatible pointer types assignment

fs_provider.c:356:12: warning: incompatible pointer types assigning to
'fs_endpoint_t *' from 'endpoint_t *'
      (aka 'struct _endpoint *') [-Wincompatible-pointer-types]
                parentep = fs_root_endpoint(path, &new_assign_path,
from_cmdline);
                         ^
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~