[System.Xml] Enable existing API under 4.0 profile too

mcs/class/System.XML/System.Xml.dll.sources

@@ -145,6 +145,7 @@ System.Xml/XmlQualifiedName.cs
 System.Xml/XmlReader.cs
 System.Xml/XmlReaderBinarySupport.cs
 System.Xml/XmlReaderSettings.cs
+System.Xml/DtdProcessing.cs
 System.Xml/XmlResolver.cs
 System.Xml/XmlSecureResolver.cs
 System.Xml/XmlSignificantWhitespace.cs

mcs/class/System.XML/System.Xml/DtdProcessing.cs

@@ -28,7 +28,7 @@
 
 
 namespace System.Xml {
-#if NET_2_1
+#if NET_2_1 || NET_4_0
 	public enum DtdProcessing 
 	{
 		Prohibit = 0,

mcs/class/System.XML/System.Xml/XmlReaderSettings.cs

@@ -60,7 +60,7 @@ namespace System.Xml
 		private ValidationType validationType;
 #endif
 		private XmlResolver xmlResolver;
-#if MOONLIGHT
+#if MOONLIGHT || NET_4_0
 		private DtdProcessing dtdProcessing;
 		private long maxCharactersFromEntities;
 		private long maxCharactersInDocument;
@@ -118,7 +118,7 @@ namespace System.Xml
 			get { return conformance; }
 			set { conformance = value; }
 		}
-#if MOONLIGHT
+#if MOONLIGHT || NET_4_0
 		public DtdProcessing DtdProcessing {
 			get { return dtdProcessing; }
 			set {