client_file: remove pure attribute from client_allow_file().

That function is not pure, it writes to error.

When marked as pure, the compiler is allowed to assume it does not do
anything to error, so it can remain NULL, which would result in an
invalid read in print_error().

src/client_file.h

@@ -35,7 +35,6 @@ struct client;
  * @param path_fs the absolute path name in filesystem encoding
  * @return true if access is allowed
  */
-G_GNUC_PURE
 bool
 client_allow_file(const struct client *client, const char *path_fs,
 		  GError **error_r);