solfege: fix run

It was failing with:

    Traceback (most recent call last):
      File "/nix/store/…-solfege-3.22.2/bin/.solfege-wrapped", line 55, in <module>
        from solfege import presetup
      File "/nix/store/…-solfege-3.22.2/share/solfege/solfege/presetup.py", line 22, in <module>
        import gtk
    ImportError: No module named gtk