Add golang.org/x/text/unicode/norm from x/text git rev a7c02369.

Needed by net/http for IDNA normalization.

Updates #13835

Change-Id: I8b024e179d573f2b093c209a4b9e4f71f7d4a1f2
Reviewed-on: https://go-review.googlesource.com/29859
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>