Forward "include_dirs" to executable target in "ios_app" template.

Forward "include_dirs" to executable target in "ios_app" template.

Many unit tests targets sets "include_dirs" variable. As the "test"
template uses "ios_app" on iOS, the content needs to be forwarded
to the "executable" target since it is the one doing the compilation.

Fixes the following error when running "gn gen" for iOS after adding
//net:net_unittests to the list of targets in //BUILD.gn:

ERROR at //ui/events/BUILD.gn:360:18: Assignment had no effect.
  include_dirs = [ "//testing/gmock/include" ]
                 ^---------------------------
You set the variable "include_dirs" here and it was unused before it went
out of scope.

BUG=459705

Review URL: https://codereview.chromium.org/1280983003

Cr-Commit-Position: refs/heads/master@{#342596}