Ensure to add the loudnorm_impl.h in the pypi source distribution

Thanks!

MANIFEST.in

@@ -3,5 +3,5 @@ recursive-include av *.pyx *.pxd
 recursive-include docs *.rst *.py
 recursive-include examples *.py
 recursive-include include *.pxd *.h
-recursive-include src/av *.c
+recursive-include src/av *.c *.h
 recursive-include tests *.py