fbpanel: correctly apply -Wno-error

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

···
   '';
 
   makeFlags = [ "V=1" ];
-  NIX_CFLAGS_COMPILE = [
+
+  env.NIX_CFLAGS_COMPILE = toString [
     "-Wno-error"
+    "-Wno-error=incompatible-pointer-types" # not implied by -Wno-error
     "-I${gdk-pixbuf-xlib.dev}/include/gdk-pixbuf-2.0"
   ];