[Foundation] Fix signature for NSLayoutConstraint.[First|Second]Anchor in .NET. (#13750)

src/Foundation/NSLayoutConstraint.cs

@@ -118,7 +118,7 @@ namespace UIKit
 			return Create ((INativeObject) view1, attribute1, relation, view2, attribute2, multiplier, constant);
 		}
 
-#if !MONOMAC || XAMCORE_4_0
+#if !MONOMAC || NET
 #if !NET
 		[iOS (10, 0)]
 #endif
@@ -134,7 +134,7 @@ namespace UIKit
 		{
 			return Runtime.GetNSObject<NSLayoutAnchor<AnchorType>> (_SecondAnchor ());
 		}
-#endif // !MONOMAC || XAMCORE_4_0
+#endif // !MONOMAC || NET
 	}
 }
 

src/xkit.cs

@@ -2325,7 +2325,7 @@ namespace UIKit {
 		[Mac (10, 12)]
 		[iOS (10,0), TV (10,0)]
 		[Export ("firstAnchor", ArgumentSemantic.Copy)]
-#if MONOMAC && !XAMCORE_4_0
+#if MONOMAC && !NET
 		NSLayoutAnchor<NSObject> FirstAnchor { get; }
 #else
 		[Internal]
@@ -2335,7 +2335,7 @@ namespace UIKit {
 		[Mac (10, 12)]
 		[iOS (10,0), TV (10,0)]
 		[Export ("secondAnchor", ArgumentSemantic.Copy)]
-#if MONOMAC && !XAMCORE_4_0
+#if MONOMAC && !NET
 		[NullAllowed]
 		NSLayoutAnchor<NSObject> SecondAnchor { get; }
 #else