iptsd: rename systemd build option to service_manager=systemd

Fixes build with meson 0.60

pkgs/applications/misc/iptsd/default.nix

···
       --replace "install_dir: rulesdir" "install_dir: datadir" \
   '';
   mesonFlags = [
-    "-Dsystemd=true"
+    "-Dservice_manager=systemd"
     "-Dsample_config=false"
     "-Ddebug_tool=false"
   ];