Add wxIOS wxSystemSettingsNative after the removal of Carbon.

Building for iOS used the carbon version of wxSystemSettingsNative which included ifdefs to build with iOS. This adds a wxIOS specific settings.mm file to provide an iOS stub implementation.

build/bakefiles/files.bkl

@@ -2478,6 +2478,7 @@ IMPORTANT: please read docs/tech/tn0016.txt before modifying this file!
     src/osx/iphone/utils.mm
     src/osx/iphone/window.mm
     src/osx/cocoa/stdpaths.mm
+    src/osx/iphone/settings.mm
 </set>
 
 <set var="OSX_IPHONE_HDR" hints="files">

build/files

@@ -2109,6 +2109,7 @@ OSX_IPHONE_SRC =
     src/osx/iphone/msgdlg.mm
     src/osx/iphone/nonownedwnd.mm
     src/osx/iphone/scrolbar.mm
+    src/osx/iphone/settings.mm
     src/osx/iphone/slider.mm
     src/osx/iphone/stattext.mm
     src/osx/iphone/textctrl.mm