misc/ios: bump -mios-version-min

Recent Xcode versions started to complain about the current min
version:

ld: warning: OS version (6.0.0) too small, changing to 7.0.0

Change-Id: Ieb525dd3e57429fe226b9d30d584b073c5e4768c
Reviewed-on: https://go-review.googlesource.com/c/go/+/204663


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>