third_party: rename build_files to $reponame.bzl

This enables IDEs to correctly identify the filetype

Change-Id: I9e2644514f001f94abe7da92c332f95889a34380
Reviewed-on: https://review.monogon.dev/c/monogon/+/3823
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/third_party/musl/musl.bzl b/third_party/musl/musl.bzl
new file mode 100644
index 0000000..9fbdeaf
--- /dev/null
+++ b/third_party/musl/musl.bzl
@@ -0,0 +1 @@
+filegroup(name = "all", srcs = glob(["**"]), visibility = ["//visibility:public"])