Bug 1215526 - part 3 - make preprocessor.preprocessor report all included files; r=glandium

For GENERATED_FILES scripts that want to report dependencies, this
change makes it easy to use |preprocess|, rather than having to
construct and use |Preprocessor| manually.

python/mozbuild/mozbuild/preprocessor.py

@@ -785,6 +785,7 @@ def preprocess(includes=[sys.stdin], defines={},
     for f in includes:
         with open(f, 'rU') as input:
             pp.processFile(input=input, output=output)
+    return pp.includes
 
 
 # Keep this module independently executable.