example: Don't import build_dso and build_ext

- they are not used in the example
- they are injected as command classes by setuptools_dso.setup itself

example/setup.py

 #!/usr/bin/env python
 
-from setuptools_dso import DSO, Extension, build_dso, build_ext, setup
+from setuptools_dso import DSO, Extension, setup
 
 dso = DSO('dsodemo.lib.demo', ['src/foo.c', 'src/bar.cpp'],
     define_macros = [('BUILD_FOO', None)],