depaware.txt: update

Upgrading staticcheck upgraded golang.org/x/sync
(one downside of mixing our tools in with our regular go.mod),
which introduced a new dependency via
https://go-review.googlesource.com/c/sync/+/251677

That CL could and probably should be written without runtime/debug,
but it's not clear to me that that is better at this moment
than simply accepting the additional package as a dependency.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/tailscale/depaware.txt

@@ -192,6 +192,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         reflect                                                      from crypto/x509+
         regexp                                                       from github.com/coreos/go-iptables/iptables+
         regexp/syntax                                                from regexp
+        runtime/debug                                                from golang.org/x/sync/singleflight
         runtime/pprof                                                from tailscale.com/log/logheap+
         sort                                                         from compress/flate+
         strconv                                                      from compress/flate+