allow modification on retrieved code_props.

The logic to compute the %printer or %destructor to used (i.e., a
code_props) is implemented twice: one, of course, in
symbol_code_props_get, and another time in symbol_check_defined to
record the fact that a code_props is used (so that we can reported
unused ones).  Let the former use the latter.

I would probably use "mutable" in C++ and keep these guys const,
but this is C.  And casting away constness triggers warnings.

* src/scan-code.h, src/scan-code.l (code_props_none): Is not const.
* src/symtab.h, src/symtab.c (symbol_code_props_get): The symbol
is not const.
(symbol_check_defined): Use it.

src/scan-code.h

@@ -103,7 +103,7 @@ void code_props_none_init (code_props *self);
   }
 
 /** Initialized by \c CODE_PROPS_NONE_INIT with no further modification.  */
-extern code_props const code_props_none;
+extern code_props code_props_none;
 
 /**
  * \pre