libffi.recipe: Don't iterate over None universal archs

Recipe 'libffi' failed at the build step 'post_install'
Traceback (most recent call last):
  File "./cerbero/cerbero/build/oven.py", line 145, in _cook_recipe
    stepfunc()
  File "./recipes/libffi.recipe", line 30, in post_install
    for arch in self.config.universal_archs:
TypeError: 'NoneType' object is not iterable