Exclude file_posix.cc from iOS build of net/ target

Move the sources! section out of link_settings, where it was
being ignored.

BUG=330074

Review URL: https://codereview.chromium.org/150973007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250961 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250961