nyan: enable and fix strictDeps build

Signed-off-by: Ethan Carter Edwards <[e-mail]>

pkgs/by-name/ny/nyan/package.nix

···
     hash = "sha256-BtvMZaQutcPqTSCN5gxYUU3CQTyCns1ldkcnjwJOFa8=";
   };
 
+  strictDeps = true;
+
   nativeBuildInputs = [
     clang
     cmake
+    flex
   ];
 
   buildInputs = [