Dependencies for libsodium-1.0.18.3

Sometimes Cabal doesn't see build tools listed in build-tool-depends if they are installed by means other than cabal-install (e.g., Nix). In those cases, not even mentioning the tools in build-tool-depends prevents cabal-install from attempting to reinstall them. This is an automatic flag, so in theory Cabal will figure out what it needs to do.