Fix crash when reading background events for MFi controllers on macOS

Enabling GCController.shouldMonitorBackgroundEvents to read background events
for MFi controllers before receiving the first GCControllerDidConnectNotification
is apparently a no-go on macOS (12.3.1 for me), and would crash on attempt.
Apple's documentation is... not great, and doesn't point this out.

This waits for IOS_AddMFIJoystickDevice() to get called down the chain from GCControllerDidConnectNotification, and enables GCController.shouldMonitorBackgroundEvents
if it hadn't been already.

On iOS and tvOS, GCController.shouldMonitorBackgroundEvents is ignored, so
there's no need to check their versions.

src/joystick/iphoneos/SDL_mfijoystick.m

@@ -180,6 +180,10 @@ IsControllerXbox(GCController *controller)
 static BOOL
 IOS_AddMFIJoystickDevice(SDL_JoystickDeviceItem *device, GCController *controller)
 {
+    if ((@available(macOS 11.3, *)) && !GCController.shouldMonitorBackgroundEvents) {
+        GCController.shouldMonitorBackgroundEvents = YES;
+    }
+
     Uint16 *guid16 = (Uint16 *)device->guid.data;
     Uint16 vendor = 0;
     Uint16 product = 0;
@@ -588,10 +592,6 @@ IOS_JoystickInit(void)
             return 0;
         }
 
-        if (@available(macOS 11.3, iOS 14.5, tvOS 14.5, *)) {
-            GCController.shouldMonitorBackgroundEvents = YES;
-        }
-
         /* For whatever reason, this always returns an empty array on
          macOS 11.0.1 */
         for (GCController *controller in [GCController controllers]) {