Turn some license strings into lib.licenses values

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

@@ -21,7 +21,7 @@ stdenv.mkDerivation {
   meta = {
     homepage = https://github.com/tom5760/usermount;
     description = "A simple tool to automatically mount removable drives using UDisks2 and D-Bus.";
-    license = "MIT";
+    license = stdenv.lib.licenses.mit;
     platforms = stdenv.lib.platforms.linux;
     maintainers = with stdenv.lib.maintainers; [ the-kenny ];
   };