iOS: use iOS style by default on iOS

When running an application on iOS, use the iOS
style by default, rather than Basic.

Change-Id: I355277f20e25720f899806ea40ed6af0da1e3033
Reviewed-by: Doris Verria <doris.verria@qt.io>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

src/quickcontrols2/doc/src/qtquickcontrols2-styles.qdoc

@@ -70,6 +70,7 @@
 
     \list
     \li Android: \l {Material Style}
+    \li iOS: \l {iOS Style}
     \li Linux: \l {Fusion Style}
     \li macOS: \l {macOS Style}
     \li Windows: \l {Windows Style}

src/quickcontrols2/qquickstyle.cpp

@@ -170,6 +170,8 @@ struct QQuickStyleSpec
             style = QLatin1String("Material");
 #elif defined(Q_OS_LINUX)
             style = QLatin1String("Fusion");
+#elif defined(Q_OS_IOS)
+            style = QLatin1String("iOS");
 #endif
             if (!style.isEmpty())
                 qCDebug(lcQtQuickControlsStyle) << "using" << style << "as a default";