support finding libzstd through pkg-config

[nektro]

the config is setup already for libsodium but not zstd

https://github.com/facebookincubator/fizz/blob/main/build/fbcode_builder/CMake/FindSodium.cmake
vs
https://github.com/facebookincubator/fizz/blob/main/build/fbcode_builder/CMake/FindZstd.cmake