udev: set to libudev-zero on static linux

systemd does not support being built statically[1], which means that,
prior to this patch, nothing that depends on udev can be built
statically.  libudev-zero is a daemonless implementation of libudev,
that supports being built statically.  So, by setting udev to be
libudev-zero when systemd is unavailable, we can get static builds of
packages like linuxPackages.usbip.

[1]: systemd/systemd#20600 (comment)

pkgs/top-level/all-packages.nix

@@ -26249,7 +26249,9 @@ with pkgs;
   };
 
 
-  udev = systemd; # TODO: change to systemdMinimal
+  udev =
+    if (with stdenv.hostPlatform; isLinux && isStatic) then libudev-zero
+    else systemd; # TODO: change to systemdMinimal
 
   systemd-wait = callPackage ../os-specific/linux/systemd-wait { };