man-pages: add license

pkgs/data/documentation/man-pages/default.nix

···
     description = "Linux development manual pages";
     homepage = https://www.kernel.org/doc/man-pages/;
     repositories.git = http://git.kernel.org/pub/scm/docs/man-pages/man-pages;
+    license = licenses.gpl2Plus;
     platforms = with platforms; unix;
     priority = 30; # if a package comes with its own man page, prefer it
   };