[ue4] Added commented build line in plugin Build.cs for UE 4.20. Updated C++ sample.

spine-ue4/Plugins/SpinePlugin/Source/SpinePlugin/SpinePlugin.Build.cs

@@ -10,6 +10,8 @@ namespace UnrealBuildTool.Rules
             PrivateIncludePaths.AddRange(new string[] { "SpinePlugin/Private", "SpinePlugin/Public/spine-cpp/include" });
             PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "RHI", "RenderCore", "ShaderCore", "ProceduralMeshComponent", "UMG", "Slate", "SlateCore" });		
 			Definitions.Add("SPINE_UE4");
+			// In Unreal 4.20, comment the above line, uncomment the below line
+			// PublicDefinitions.Add("SPINE_UE4");
 		}
 	}
 }