protoproxy: add proxy_read_timeout to IPv6 too

I605ab157a3723da5663bb8fa978f16a42d67d4f1 changed the timeout, but only
for the SSL part. Fix the IPv6 part of the config as well, short-lived
as it may be.

Change-Id: I13c95d1881c64690fb1d1bd009e9410049eb5962