[bazel] Convert buildifier to a dev dependency

These are dependencies that are only needed when working on the
OpenTitan modules itself and not needed by modules depending on
OpenTitan.

Signed-off-by: James Wainwright <james.wainwright@lowrisc.org>

MODULE.bazel

@@ -4,9 +4,17 @@
 
 module(name = "lowrisc_opentitan")
 
+# Dependencies:
 bazel_dep(name = "bazel_skylib", version = "1.7.1")
-bazel_dep(name = "buildifier_prebuilt", version = "6.4.0")
 
+# Dev dependencies:
+bazel_dep(
+    name = "buildifier_prebuilt",
+    version = "6.4.0",
+    dev_dependency = True,
+)
+
+# Includes:
 include("//third_party/rust:rust.MODULE.bazel")
 
 include("//third_party/python:python.MODULE.bazel")
@@ -15,6 +23,7 @@ include("//third_party/lint:lint.MODULE.bazel")
 
 include("//third_party/google:google.MODULE.bazel")
 
+# Repository rules:
 bitstreams_repo = use_repo_rule("//rules:bitstreams.bzl", "bitstreams_repo")
 
 bitstreams_repo(name = "bitstreams")