When autoloading a completion, also autoload the function

Fixes a case where a --wraps declaration would be missed
because the function would not be loaded. Fixes #2466.

src/function.h

@@ -130,6 +130,9 @@ void function_set_desc(const wcstring &name, const wcstring &desc);
 */
 int function_exists(const wcstring &name);
 
+/** Attempts to load a function if not yet loaded. This is used by the completion machinery. */
+void function_load(const wcstring &name);
+
 /**
    Returns true if the function with the name name exists, without triggering autoload.
 */