fbset: fix cross build

Otherwise `nix build .#pkgsCross.riscv64.fbset` fails with

> bash: line 1: gcc: command not found

pkgs/by-name/fb/fbset/package.nix

@@ -23,6 +23,8 @@ stdenv.mkDerivation (finalAttrs: {
     flex
   ];
 
+  makeFlags = [ "CC=${stdenv.cc.targetPrefix}cc" ];
+
   installFlags = [ "DESTDIR=$(out)" ];
 
   passthru = {