api: add os.(*File).SyscallConn to go1.12.txt
api: add os.(*File).SyscallConn to go1.12.txt

Fixes #29507

Change-Id: I8cf52e4b89fd28126f252757260d07a31d9dad61
Reviewed-on: https://go-review.googlesource.com/c/155933
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,josharian/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,golang/go,akutz/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,josharian/go,nawawi/go,josharian/go,josharian/go,nawawi/go,akutz/go,josharian/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,golang/go,golang/go,josharian/go,golang/go,cloudflare/go,josharian/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go