nixos/pam: Fix wrong string concatenation

Regression introduced by 1010271c

.

This caused the line after using the loginuid module to be concatenated
with the next line without a newline.

In turn this has caused a lot of the NixOS VM tests to either run very
slowly (because of constantly hitting PAM errors) or simply fail.

I have tested this only with one of the failing NixOS tests.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>