iproute: 5.5.0 -> 5.6.0

"Not a lot of changes in this release, most are related to fixing output
formatting and documentation." [0]

File changes (additions/removals):
+share/man/man8/tc-ets.8.gz
+share/man/man8/tc-fq_pie.8.gz

nix path-info -S:
5.5.0 51509616
5.6.0 51528680

[0]: https://marc.info/?l=linux-netdev&m=158585608413591

pkgs/os-specific/linux/iproute/default.nix

@@ -2,11 +2,11 @@
 
 stdenv.mkDerivation rec {
   pname = "iproute2";
-  version = "5.5.0";
+  version = "5.6.0";
 
   src = fetchurl {
     url = "mirror://kernel/linux/utils/net/${pname}/${pname}-${version}.tar.xz";
-    sha256 = "0ywg70f98wgfai35jl47xzpjp45a6n7crja4vc8ql85cbi1l7ids";
+    sha256 = "14j6n1bc09xhq8lxs40vfsx8bb8lx12a07ga4rsxl8vfrqjhwnqv";
   };
 
   preConfigure = ''