mscgen: clean up build

1. Remove freetype dependency:
    - was not being used by build
    - trying to enable freetype gives nontrivial error
      'gdoTextWidth: Problem doing text layout (GDFONTPATH=)'

2. Correctly link manpages by specifying 'outputs'.

3. Separate nativeBuildInputs from buildInputs

Signed-off-by: Sirio Balmelli <sirio@b-ad.ch>