nixos/sshd: accept path-typed options

When the 'banner' option was removed in favour of settings.Banner,
it was made a path-typed option, but the current mkValueString doesn't
accept path-typed values so assigning a file to this option results in:
`error: unsupported type path: <store path>`

By adding the appropriate handling for mkValueString, this works as
expected.

nixos/modules/services/networking/ssh/sshd.nix

@@ -26,6 +26,8 @@ let
           toString v
         else if lib.isString v then
           v
+        else if lib.isPath v then
+          v
         else if true == v then
           "yes"
         else if false == v then