Add providers.bzl to `shell/private:private_bzl` (#57)

This file is missing in the bzl_library target which makes starlark
generation fail for example:

```
Stardoc documentation generation failed: File /home/runner/.cache/bazel/_bazel_runner/c91220d87f26118b19246641d5266d21/sandbox/processwrapper-sandbox/251/execroot/rules_haskell/bazel-out/k8-opt-exec-2B5CBBC6/bin/docs/defs_metadata_stardoc.runfiles/rules_shell/shell/private/sh_executable.bzl imported ':providers.bzl', yet /home/runner/.cache/bazel/_bazel_runner/c91220d87f26118b19246641d5266d21/sandbox/processwrapper-sandbox/251/execroot/rules_haskell/bazel-out/k8-opt-exec-2B5CBBC6/bin/docs/defs_metadata_stardoc.runfiles/rules_shell/shell/private/providers.bzl was not found.
```

shell/private/BUILD

diff --git a/shell/private/BUILD b/shell/private/BUILD
index d0eebed..23eb387 100644
--- a/shell/private/BUILD
+++ b/shell/private/BUILD
@@ -3,6 +3,7 @@
 bzl_library(
     name = "private_bzl",
     srcs = [
+        "providers.bzl",
         "sh_binary.bzl",
         "sh_executable.bzl",
         "sh_library.bzl",