Restore [Require]

Author: grendello

src/Xamarin.Android.Build.Tasks/Tasks/GeneratePackageManagerJava.cs

@@ -63,7 +63,7 @@ public override bool RunTask ()
 		public bool EnablePreloadAssembliesDefault { get; set; }
 
 		// This property should be required but it will require modifying `monodroid` first
-		//[Required]
+		[Required]
 		public string AndroidBinUtilsDirectory { get; set; }
 
 		public bool EnableMarshalMethods { get; set; }