Specify LayoutKind for structs

src/Gir.Tests/ImportantGeneratables/GObject.cs

@@ -365,6 +365,7 @@ public void GTypeStructsAreInternal ()
 
 namespace GObject
 {
+	[StructLayout(LayoutKind.Sequential)]
 	internal struct ObjectClass
 	{
 		TypeClass GTypeClass;

src/Gir.Tests/ImportantGeneratables/List.cs

@@ -15,6 +15,7 @@ public void ListIsGenerated ()
 
 namespace GLib
 {
+	[StructLayout(LayoutKind.Sequential)]
 	public struct List
 	{
 		IntPtr Data;

src/Gir.Tests/RecordTests.cs

@@ -18,6 +18,7 @@ public void TestRecordIsGenerated ()
 namespace GLib
 {
 	///<summary>Contains the public fields of a GByteArray.</summary>
+	[StructLayout(LayoutKind.Sequential)]
 	public struct ByteArray
 	{
 		///<summary>

src/Gir/Generation/Record.cs

@@ -12,6 +12,7 @@ public void Generate (GenerationOptions opts)
 				if (!string.IsNullOrEmpty (GLibIsGTypeStructFor))
 					access = "internal";
 
+				writer.WriteLine ("[StructLayout(LayoutKind.Sequential)]");
 				writer.WriteLine ($"{access} struct {Name}");
 				writer.WriteLine ("{");