maintainers: add swesterfeld

Signed-off-by: Stefan Westerfeld <[e-mail]>

maintainers/maintainer-list.nix

···
     github = "sweenu";
     githubId = 7051978;
   };
+  swesterfeld = {
+    email = "[e-mail]";
+    github = "swesterfeld";
+    githubId = 14840066;
+    name = "Stefan Westerfeld";
+  };
   swflint = {
     email = "[e-mail]";
     github = "swflint";