add nix.packages to stack.yaml

postgrest depends on postgres and zlib. When using stack's nix
integration, these dependencies must be specified explicitly.

Note that `--nix` must be passed to stack, or `nix.enable:true` must
be set in `stack.yaml` to enable using nix while building.

stack.yaml

@@ -5,3 +5,5 @@ extra-deps:
   - hasql-transaction-0.4.5.1
 ghc-options:
   postgrest: -O2 -Werror -Wall -fwarn-identities
+nix:
+  packages: [postgresql, zlib]