make vmodtool reject arguments of type VOID

VOID is only valid as a return type. If VOID was used as an argument,
the C compiler would bail out, yet we should better check this early.

Looking through CTYPES, VOID seems to be the only type not valid for
arguments, thus the explicit check.

lib/libvcc/vmodtool.py

@@ -359,6 +359,8 @@ class ProtoType(object):
                 self.argstruct = True
             else:
                 t = arg(wl, names, st.vcc.enums, ',')
+            if t.vt == 'VOID':
+                err("arguments can not be of type '%s'" % t.vt, warn=False)
             if t.nm is None:
                 t.nm2 = "arg%d" % n
             else: