Cannot traverse nodes using NodeVisitor, FuncDeclExt is not iterable

Using `GnuCParser()` and subclassing `c_ast.NodeVisitor`, I cannot traverse the nodes as in the FuncDefs example in https://github.com/eliben/pycparser/blob/master/examples/func_defs.py

It results in the following error:
```
Traceback (most recent call last):
[...]
File "main_script.py", line 77, in verify
FuncDefVisitor().visit(self.ast)
File "/path/venv/lib/python3.6/site-packages/pycparser/c_ast.py", line 158, in visit
return visitor(node)
File "/path/venv/lib/python3.6/site-packages/pycparser/c_ast.py", line 165, in generic_visit
self.visit(c)
File "/path/venv/lib/python3.6/site-packages/pycparser/c_ast.py", line 158, in visit
return visitor(node)
File "/path/venv/lib/python3.6/site-packages/pycparser/c_ast.py", line 165, in generic_visit
self.visit(c)
File "/path/venv/lib/python3.6/site-packages/pycparser/c_ast.py", line 158, in visit
return visitor(node)
File "/path/venv/lib/python3.6/site-packages/pycparser/c_ast.py", line 164, in generic_visit
for c in node:
TypeError: 'FuncDeclExt' object is not iterable
```