slim: add license

see issue #43716

+1
+1
pkgs/applications/display-managers/slim/default.nix
··· 50 50 meta = { 51 51 homepage = https://sourceforge.net/projects/slim.berlios/; # berlios shut down; I found no replacement yet 52 52 platforms = stdenv.lib.platforms.linux; 53 + license = stdenv.lib.licenses.gpl2; 53 54 }; 54 55 }