opencascade-occt: fixup build by avoiding warning

Probably after freetype update.
https://hydra.nixos.org/build/273837263/nixlog/13/tail

pkgs/development/libraries/opencascade-occt/default.nix

@@ -39,6 +39,8 @@ stdenv.mkDerivation rec {
     libXi
   ] ++ lib.optional stdenv.hostPlatform.isDarwin darwin.apple_sdk.frameworks.Cocoa;
 
+  NIX_CFLAGS_COMPILE = [ "-fpermissive" ];
+
   meta = with lib; {
     description = "Open CASCADE Technology, libraries for 3D modeling and numerical simulation";
     homepage = "https://www.opencascade.org/";