dosfstools: add license

see issue #43716

pkgs/tools/filesystems/dosfstools/default.nix

···
     repositories.git = git://daniel-baumann.ch/git/software/dosfstools.git;
     homepage = https://www.daniel-baumann.ch/software/dosfstools/;
     platforms = stdenv.lib.platforms.linux ++ stdenv.lib.platforms.darwin;
+    license = stdenv.lib.licenses.gpl3;
   };
 }