bc: flex is also a runtime dep

This is one of the rare times when flex is not just needed at build
time. When cross compiling, we will need 2 versions of flex - one for
the local system and one for the cross system.

pkgs/tools/misc/bc/default.nix

@@ -21,7 +21,7 @@ stdenv.mkDerivation rec {
     # Libraries for build
     buildPackages.readline buildPackages.ncurses
   ];
-  buildInputs = [ readline ];
+  buildInputs = [ readline flex ];
 
   doCheck = true; # not cross