time: add ios build constraint to zoneinfo_ios.go
time: add ios build constraint to zoneinfo_ios.go

This allows to build the time package using a Go version which doesn't
know GOOS=ios yet.

Change-Id: Ib1b00687432f3309bac8fd8bf5c02b9c62f049a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/261362
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go