Add BUILD file for bitflags kleaf build

Test:
touch external/rust/crates/bitflags/WORKSPACE && \
b build @bitflags \
    --override_repository=bitflags=external/rust/crates/bitflags/
Bug: 304282799

Change-Id: Icc7288136dc7e03b1fe7b7bdf243cc12f522b90b

BUILD

diff --git a/BUILD b/BUILD
new file mode 100644
index 0000000..156cf27
--- /dev/null
+++ b/BUILD
@@ -0,0 +1,36 @@
+load("@rules_license//rules:license.bzl", "license")
+load("@rules_license//rules:license_kind.bzl", "license_kind")
+load("@rules_rust//rust:defs.bzl", "rust_library")
+
+package(
+    default_applicable_licenses = [":license"],
+    default_visibility = ["//visibility:public"],
+)
+
+license(
+    name = "license",
+    license_kinds = [
+        ":SPDX-license-identifier-Apache-2.0",
+        ":SPDX-license-identifier-MIT",
+    ],
+    license_text = "LICENSE-APACHE",
+    visibility = [":__subpackages__"],
+)
+
+license_kind(
+    name = "SPDX-license-identifier-Apache-2.0",
+    conditions = ["notice"],
+    url = "https://spdx.org/licenses/Apache-2.0.html",
+)
+
+license_kind(
+    name = "SPDX-license-identifier-MIT",
+    conditions = ["notice"],
+    url = "",
+)
+
+rust_library(
+    name = "bitflags",
+    srcs = glob(["**/*.rs"]),
+    edition = "2021",
+)