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.

pkgs/os-specific/linux/lttng-modules/default.nix

@@ -23,7 +23,9 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     description = "Linux kernel modules for LTTng tracing";
     homepage = http://lttng.org/;
-    license = with licenses; [ lgpl21 gpl2 mit ];
+    # TODO: Add "mit" to the license list once the license attr set vs string
+    # decision has been made. (Having "mit" there breaks hydra evaluation.)
+    license = with licenses; [ lgpl21 gpl2 ];
     platforms = platforms.linux;
     maintainers = [ maintainers.bjornfor ];
   };