qstandardpaths_ios: use fallback for DesktopLocation

NSDesktopDirectory points to a non-existing write-protected path inside
the app sandbox. According to QSP documentation, we should fall back to
use the home directory instead.

Change-Id: I2c370af7758ac043eddcff84aa287eacc754ae38
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/corelib/io/qstandardpaths_ios.mm

@@ -55,9 +55,6 @@ QString QStandardPaths::writableLocation(StandardLocation type)
     QString location;
 
     switch (type) {
-    case DesktopLocation:
-        location = pathForDirectory(NSDesktopDirectory);
-        break;
     case DocumentsLocation:
         location = pathForDirectory(NSDocumentDirectory);
         break;
@@ -82,6 +79,7 @@ QString QStandardPaths::writableLocation(StandardLocation type)
     case TempLocation:
         location = QString::fromNSString(NSTemporaryDirectory());
         break;
+    case DesktopLocation:
     case HomeLocation:
         location = bundlePath();
         break;