[ios] specify plist-file for settings bundle

[frederoni]

Fixes #6947

Did this start with Xcode 8.1 or later?
Root.plist is usually picked up automatically from the settings bundle. Not sure why it doesn't all of a sudden.

@kkaefer 👀

[mention-bot]

@frederoni, thanks for your PR! By analyzing the history of the files in this pull request, we identified @1ec5, @boundsj and @friedbunny to be potential reviewers.

[1ec5]

Unit tests are failing to compile on iOS (but not macOS, interestingly enough):

▸ Compiling MGLBackgroundStyleLayer.mm

❌  /Users/vagrant/git/platform/darwin/src/NSExpression+MGLAdditions.h:3:10: 'mbgl/style/filter.hpp' file not found

#include <mbgl/style/filter.hpp>
         ^


▸ Compiling NSComparisonPredicate+MGLAdditions.mm

❌  /Users/vagrant/git/platform/darwin/src/NSComparisonPredicate+MGLAdditions.h:3:10: 'mbgl/style/filter.hpp' file not found

#include <mbgl/style/filter.hpp>
         ^


xcode test exit code: 65
xcode test failed, error: exit status 65

[1ec5]

Did this start with Xcode 8.1 or later?

That must be what introduced the failure.

Feel free to merge once we figure out what’s going on with the iOS unit tests.