Fix build on macOS: Apple changed their minds if null is allowed

qcorewlanengine.mm:88:37: error: null passed to a callee that requires a non-null argument [-Werror,-Wnonnull]
    currentInterface = [CWInterface interfaceWithName:nil];
                                    ^                 ~~~

Change-Id: Ie7d159a97176ca53161423cfcddba547e735d5b3
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/plugins/bearer/corewlan/qcorewlanengine.mm

@@ -79,7 +79,7 @@ extern "C" { // Otherwise it won't find CWKeychain* symbols at link time
     [locker lock];
     QMacAutoReleasePool pool;
     notificationCenter = [NSNotificationCenter defaultCenter];
-    currentInterface = [CWInterface interfaceWithName:nil];
+    currentInterface = [CWInterface interface];
     [notificationCenter addObserver:self selector:@selector(notificationHandler:) name:CWPowerDidChangeNotification object:nil];
     [locker unlock];
     return self;