validate:launcher: Install config.py for non-development mode

Running installed gst-validate-launcher aborted with the following error.

File "lib/gst-validate-launcher/python/launcher/baseclasses.py", line 28, in <module>
  import config
ImportError: No module named config

This is because config.py is added but not installed
in ba6d209b3fd062f4e6bd889f81f1213cc12339ec.

https://bugzilla.gnome.org/show_bug.cgi?id=747087

validate/launcher/Makefile.am

@@ -11,7 +11,8 @@ launcher_PYTHON = \
 	main.py  \
 	httpserver.py  \
 	RangeHTTPServer.py  \
-	utils.py
+	utils.py  \
+	config.py
 
 clean-local:
 	rm -rf *.pyc *.pyo