Add internal ctor to internal interop readonly struct for warning CS0649

It is being created by Marshal methods. Adding the ctor makes the compiler warning go away without having to pragma disable them.

LibVLCSharp/Shared/Structures/MediaDiscovererDescription.cs

@@ -4,6 +4,13 @@ namespace LibVLCSharp.Shared
 {
     internal readonly struct MediaDiscovererDescriptionStructure
     {
+        internal MediaDiscovererDescriptionStructure(IntPtr name, IntPtr longName, MediaDiscovererCategory category)
+        {
+            Name = name;
+            LongName = longName;
+            Category = category;
+        }
+
         internal readonly IntPtr Name;
         internal readonly IntPtr LongName;
         internal readonly MediaDiscovererCategory Category;

LibVLCSharp/Shared/Structures/MediaSlave.cs

@@ -4,6 +4,13 @@ namespace LibVLCSharp.Shared
 {
     internal readonly struct MediaSlaveStructure
     {
+        internal MediaSlaveStructure(IntPtr uri, MediaSlaveType type, uint priority)
+        {
+            Uri = uri;
+            Type = type;
+            Priority = priority;
+        }
+
         internal readonly IntPtr Uri;
         internal readonly MediaSlaveType Type;
         internal readonly uint Priority;

LibVLCSharp/Shared/Structures/MediaTrack.cs

@@ -21,6 +21,11 @@ namespace LibVLCSharp.Shared
     internal readonly struct SubtitleTrackStructure
     {
         internal readonly IntPtr Encoding;
+
+        internal SubtitleTrackStructure(IntPtr encoding)
+        {
+            Encoding = encoding;
+        }
     }
 
     /// <summary>

LibVLCSharp/Shared/Structures/RendererDescription.cs

@@ -4,6 +4,12 @@ namespace LibVLCSharp.Shared
 {
     internal readonly struct RendererDescriptionStructure
     {
+        internal RendererDescriptionStructure(IntPtr name, IntPtr longName)
+        {
+            Name = name;
+            LongName = longName;
+        }
+
         internal readonly IntPtr Name;
         internal readonly IntPtr LongName;
     }