feat(third_party): Add missing Quicklisp packages for Gemma

Gemma depends on cl-prevalence, which isn't in the nixpkgs Quicklisp
snapshot.

This adds the package and its dependencies to the overlay.

third_party/common_lisp/quicklisp-to-nix-output/s-xml.nix

@@ -0,0 +1,27 @@
+args @ { fetchurl, ... }:
+rec {
+  baseName = ''s-xml'';
+  version = ''20150608-git'';
+
+  parasites = [ "s-xml.examples" "s-xml.test" ];
+
+  description = ''Simple Common Lisp XML Parser'';
+
+  deps = [ ];
+
+  src = fetchurl {
+    url = ''http://beta.quicklisp.org/archive/s-xml/2015-06-08/s-xml-20150608-git.tgz'';
+    sha256 = ''0cy36wqzasqma4maw9djq1vdwsp5hxq8svlbnhbv9sq9zzys5viq'';
+  };
+
+  packageName = "s-xml";
+
+  asdFilesToKeep = ["s-xml.asd"];
+  overrides = x: x;
+}
+/* (SYSTEM s-xml DESCRIPTION Simple Common Lisp XML Parser SHA256
+    0cy36wqzasqma4maw9djq1vdwsp5hxq8svlbnhbv9sq9zzys5viq URL
+    http://beta.quicklisp.org/archive/s-xml/2015-06-08/s-xml-20150608-git.tgz
+    MD5 9c31c80f0661777c493fab683f776716 NAME s-xml FILENAME s-xml DEPS NIL
+    DEPENDENCIES NIL VERSION 20150608-git SIBLINGS NIL PARASITES
+    (s-xml.examples s-xml.test)) */