munin-node: patch to configure plugin-conf.d

Add a new patch (adding_sconfdir_munin-node.patch) to be able to
configure the location of plugin-conf.d (otherwise it has to be
configured at build time).  This patch is very similar to the
existing 'adding_servicedir_munin-node.patch'.

pkgs/servers/monitoring/munin/default.nix

@@ -68,6 +68,7 @@ stdenv.mkDerivation rec {
     # https://github.com/munin-monitoring/munin/pull/134
     ./adding_servicedir_munin-node.patch
 
+    ./adding_sconfdir_munin-node.patch
     ./preserve_environment.patch
   ];