lttng-modules: don't use licenses.mit because it breaks hydra evaluation

Hydra has these evaluation errors:
  at `nixpkgs.linuxPackages.lttngModules.i686-linux' [nixosSrc = ..., nixpkgs = ..., officialRelease = false]:
  value is an attribute set while a string was expected

because licenses.mit is an attribute set and not a string.

TODO: Licenses in pkgs/lib/licenses.nix are a mix of attr sets and
strings, this needs to be standardized.