Fix UV material slot serialization

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Graphs/UVMaterialSlot.cs

     [Serializable]
     public class UVMaterialSlot : Vector2MaterialSlot, IMayRequireMeshUV
     {
+        [SerializeField]
         private UVChannel m_Channel = UVChannel.UV0;
 
         public UVChannel channel