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/third_party/seccomp/seccomp.bzl b/third_party/seccomp/seccomp.bzl
index 782c762..6b0a9aa 100644
--- a/third_party/seccomp/seccomp.bzl
+++ b/third_party/seccomp/seccomp.bzl
@@ -78,5 +78,6 @@
         ":seccomp.h",
         "include/seccomp-syscalls.h",
     ],
+    includes = ["."],
     visibility = ["//visibility:public"],
 )