[flake8] Ignore build/ directory

That directory is temporarily generated when building for PyPi and may be present if something goes wrong with the upload.

setup.cfg

@@ -2,5 +2,5 @@
 universal = True
 
 [flake8]
-exclude = youtube_dl/extractor/__init__.py,devscripts/buildserver.py,setup.py
+exclude = youtube_dl/extractor/__init__.py,devscripts/buildserver.py,setup.py,build
 ignore = E501