ntp: 4.2.8p12 -> 4.2.8p13 (#57059)

From http://www.ntp.org/index.html:
> ntp-4.2.8p13 was released on 07 March 2019.
> It addresses 1 medium-severity security issue in ntpd, and provides 17
> other non-security fixes and 1 improvements over 4.2.8p12.

pkgs/tools/networking/ntp/default.nix

@@ -8,11 +8,11 @@ let
 in
 
 stdenv.mkDerivation rec {
-  name = "ntp-4.2.8p12";
+  name = "ntp-4.2.8p13";
 
   src = fetchurl {
     url = "https://www.eecis.udel.edu/~ntp/ntp_spool/ntp4/ntp-4.2/${name}.tar.gz";
-    sha256 = "0m04ndn0674kcf9x0aggjya07a3hlig2nlzzpwk7vmqka0mj56vh";
+    sha256 = "0f1a4fya7v5s0426nim8ydvvlcashb8hicgs9xlm76ndrz7751r8";
   };
 
   # The hardcoded list of allowed system calls for seccomp is