nixos: add uid/gid for munin

To be compatible with eb2f44c18c (Generate
/etc/passwd and /etc/group at build time). Without this you'll get this:

  $ nixos-rebuild build
  [...]
  user-thrown exception: The option `users.extraGroups.unnamed-9.1.gid' is used but not defined.

nixos/modules/misc/ids.nix

@@ -110,6 +110,7 @@
       openldap = 99;
       memcached = 100;
       cgminer = 101;
+      munin = 102;
 
       # When adding a uid, make sure it doesn't match an existing gid.
 
@@ -199,6 +200,7 @@
       haproxy = 92;
       openldap = 93;
       connman = 94;
+      munin = 95;
 
       # When adding a gid, make sure it doesn't match an existing uid.