Compile error with libstdc++ 11

[Novum]

https://github.com/mitsuba-renderer/drjit/blob/ab1bc8683049a8aadc8cc0233a26fb2ee14064bc/include/drjit/packet_intrin.h#L48

GCC 11 complains with: error: #error "Never use <bmi2intrin.h> directly; include <x86gprintrin.h> instead."

[dvicini]

Hi, We currently test only GCC 9 on the CI, so I am not surprised that GCC 11 would run into some minor issues here and there. Feel free to make a pull request with a fix. Otherwise, you could also just use Clang. We ran into issues with GCC every now and then, so pretty much everybody working on Dr.Jit and Mitsuba 3 is mostly using Clang.

[tomix1024]

I think I fixed this problem in #55 .