third_party/chrony: initialize

First pass at building chrony. Minimal functionality, notably skipped
features are:

 - PRIVDROP (requires libcap)
 - NTS (requires gnutls)

Do we need anything else?

Tested with:

    $ bazel build '@chrony//:chrony' --crosstool_top=//build/toolchain/musl-host-gcc:musl_host_cc_suite
    $ file bazel-bin/external/chrony/chrony
    bazel-bin/external/chrony/chrony: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped
    $ bazel-bin/external/chrony/chrony -v
    chronyd (chrony) version 4.1-monogon (NTP RTC SCFILTER ASYNCDNS)

Change-Id: I56ac15a23e5741c0428580268cf40ae7744078d4
Reviewed-on: https://review.monogon.dev/c/monogon/+/293
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/WORKSPACE b/WORKSPACE
index 00c6aeb..1e2d57b 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -196,6 +196,12 @@
     version = "5.2.0",
 )
 
+load("//third_party/chrony:external.bzl", "chrony_external")
+
+chrony_external(
+    name = "chrony",
+)
+
 register_toolchains("//:host_python")
 
 # python dependencies. Currently we don't use Python, but some of our deps (ie. gvisor) do expect @pydeps// to exist, even