gn: fix build with GCC 5

Technically, this code is correct, but practically a problem as it
makes it hard to read and comprehend for people that have not
completely memorised the C++ standard and its scoping rules.

It also triggers a compiler bug in GCC 5.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54430
https://bugs.gentoo.org/show_bug.cgi?id=605828

tools/gn/xcode_writer.cc:173:29: error: use of ‘target’ before deduction of ‘auto’
   for (const auto& target : target->public_deps()) {

Review-Url: https://codereview.chromium.org/2632113003
Cr-Original-Commit-Position: refs/heads/master@{#444083}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6d75d94d4a41eab2f2963e5ae0cdc0cfad4706c9
1 file changed
tree: 4a9d915d0ed08e1c17ff811810ad0e061702d2ba
  1. tools/