Merge pull request #111579 from dotlambda/libsixel-insecure

libsixel: mark as insecure

pkgs/development/libraries/libsixel/default.nix

···
     maintainers = with maintainers; [ vrthra ];
     license = licenses.mit;
     platforms = with platforms; unix;
+    knownVulnerabilities = [
+      "CVE-2020-11721" # https://github.com/saitoha/libsixel/issues/134
+      "CVE-2020-19668" # https://github.com/saitoha/libsixel/issues/136
+    ];
   };
 }