rawtherapee: fixup build

I expect this was due to gcc 12 -> 13.
It's confusing that x86_64 doesn't suffer from this but aarch64 does:
https://hydra.nixos.org/build/247470042/nixlog/1/tail

For some reason it didn't suffice to extend CMAKE_CXX_FLAGS.

pkgs/applications/graphics/rawtherapee/default.nix

@@ -79,6 +79,7 @@ stdenv.mkDerivation rec {
     "-Wno-deprecated-declarations"
     "-Wno-unused-result"
   ];
+  env.CXXFLAGS = "-include cstdint"; # needed at least with gcc13 on aarch64-linux
 
   postInstall = lib.optionalString stdenv.isDarwin ''
     mkdir -p $out/Applications/RawTherapee.app $out/bin