mkcl: add myself as a maintainer

pkgs/development/compilers/mkcl/default.nix

···
     homepage = https://common-lisp.net/project/mkcl/;
     license = stdenv.lib.licenses.lgpl2Plus;
     platforms = stdenv.lib.platforms.linux;
+    maintainers = [stdenv.lib.maintainers.tohl];
   };
 }