call: fix conference creation

Change-Id: I540d4d1331b09114005034027bbba090127304ff

src/ChooseContactVC.mm

@@ -147,8 +147,10 @@ NSInteger const MAXIMUM_TABLE_SIZE = 240;
 
 -(void)viewDidLoad {
     [super viewDidLoad];
+    if (@available(macOS 11.0, *)) {
         contactsView.style = NSTableViewStylePlain;
         callsView.style = NSTableViewStylePlain;
+    }
     [self reloadView];
 }