d/rules: fix perms of /etc/gestioip/ip_config

This seems the only way to make the file usable by gestioip.

However, this fix produces a lintian warning:
 W: gestioip: non-standard-file-perm etc/gestioip/ip_config 0664 != 0644

The Debian Policy, at 10.9 says:
  <<If necessary you may deviate from the details below. However, if you do so
  you must make sure that what is done is secure and you should try to be as
  consistent as possible with the rest of the system.>>

I think this permissions are good.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>