setup: Add dependencies for Windows

Windows apparently builds pylibfdt if swig is declared as a dependency
in the setup file. Otherwise it must be installed manually and Windows
does not sport a package manager, so this is hard for users.

This change seems to have no ill effects on Linux, at least.

So add a new setup_requires line.

Commit-notes:
This was picked up from:

devicetree-org/pylibfdt#2
END

Series-to: dtc
Series-cc: Gua Guo <[email protected]>, david

Signed-off-by: Gua Guo <[email protected]>
Signed-off-by: Simon Glass <[email protected]>

setup.py

@@ -76,6 +76,7 @@ def run(self):
     cmdclass = {'build_py' : BuildPy},
     author='Simon Glass',
     author_email='[email protected]',
+    setup_requires = ['swig'],
     description='Python binding for libfdt',
     ext_modules=[libfdt_module],
     package_dir={'': os.path.join(srcdir, 'pylibfdt')},