Some tests need the src/runtime/textflag.h file. Make sure it is
included in iOS test runs.

Change-Id: I5e0e7ebe85679686ef15a7d336f28ac9b68a587a
Reviewed-on: https://go-review.googlesource.com/40915
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>