Fix gcc no input file issues by passing dummy file

For Linux UCS2, the build was failing due to gcc complaining it could not find any files. This reproduced locally when running `./pants3 setup-py --run="bdist_wheel --py-limited-api cp36" src/python/pants:pants-packaged` on OSX.

John suggested and gave the code snippet to pass a dummy file so this no longer happens. Thanks John!

src/python/pants/BUILD

@@ -10,15 +10,15 @@ target(
 
 python_library(
   name='pants-packaged',
-  sources=[],
+  sources=['dummy.c'],
   dependencies=[
     ':version',
   ],
   provides=pants_setup_py(
     name='pantsbuild.pants',
     description='A scalable build tool for large, complex, heterogeneous repos.',
     namespace_packages=['pants', 'pants.backend'],
-    ext_modules=[('native_engine', {'sources': []})],
+    ext_modules=[('native_engine', {'sources': ['src/pants/dummy.c']})],
   ).with_binaries(
     pants='src/python/pants/bin:pants',
   )