p7zip: add license

see issue #43716

pkgs/tools/archivers/p7zip/default.nix

···
     # license = stdenv.lib.licenses.lgpl21Plus; + "unRAR restriction"
     platforms = stdenv.lib.platforms.unix;
     maintainers = [ stdenv.lib.maintainers.raskin ];
+    license = stdenv.lib.licenses.lgpl2Plus;
   };
 }