Switch maccore to xcode11 branch

mk/xamarin.mk

@@ -1,6 +1,6 @@
 ifdef ENABLE_XAMARIN
-NEEDED_MACCORE_VERSION := 12ecfd8249cab8d3867a779e5ce633f97bb3825b
-NEEDED_MACCORE_BRANCH := d16-2
+NEEDED_MACCORE_VERSION := 54d98e168c0493e05f26c0171540b2e7475b7b41
+NEEDED_MACCORE_BRANCH := xcode11
 
 MACCORE_DIRECTORY := maccore
 MACCORE_MODULE    := git@github.com:xamarin/maccore.git