rsyslog: fixup build

For some reason it now wasn't linking properly anymore:
https://hydra.nixos.org/build/321669732/nixlog/3/tail
This change should be relatively safe.

pkgs/tools/system/rsyslog/default.nix

@@ -180,6 +180,8 @@ stdenv.mkDerivation rec {
     (lib.enableFeature true "generate-man-pages")
   ];
 
+  NIX_CFLAGS_LINK = "-lz";
+
   passthru.tests = {
     nixos-rsyslogd = nixosTests.rsyslogd;
   };