[bgen] Fully qualify the RequiredMember and OptionalMember attributes. …

…Fixes #21073. (#21076)

There's a `System.Runtime.CompilerServices.RequiredMemberAttribute`
type, which may lead to ambiguous type references otherwise.

Fixes #21073.

src/bgen/Generator.cs

@@ -3926,9 +3926,9 @@ void PrintProtocolMemberAttributes (MemberInformation minfo)
 
 #if NET
 		if (minfo.is_protocol_member_required.Value) {
-			print ("[RequiredMember]");
+			print ("[global::Foundation.RequiredMember]");
 		} else {
-			print ("[OptionalMember]");
+			print ("[global::Foundation.OptionalMember]");
 		}
 #endif
 	}