[unity] Fixed compile error of previous commit 238818d on Unity 2017. See #2358.

spine-unity/Assets/Spine/Runtime/spine-unity/Components/SkeletonRenderSeparator/SkeletonRenderSeparator.cs

@@ -30,6 +30,11 @@
 #if UNITY_2018_3 || UNITY_2019 || UNITY_2018_3_OR_NEWER
 #define NEW_PREFAB_SYSTEM
 #endif
+
+#if UNITY_2018_1_OR_NEWER
+#define HAS_PROPERTY_BLOCK_QUERY
+#endif
+
 #define SPINE_OPTIONAL_RENDEROVERRIDE
 
 using System.Collections.Generic;
@@ -207,7 +212,11 @@ namespace Spine.Unity {
 			int rendererCount = partsRenderers.Count;
 			if (rendererCount <= 0) return;
 
+#if HAS_PROPERTY_BLOCK_QUERY
 			bool assignPropertyBlock = this.copyPropertyBlock && mainMeshRenderer.HasPropertyBlock();
+#else
+			bool assignPropertyBlock = this.copyPropertyBlock;
+#endif
 			if (assignPropertyBlock)
 				mainMeshRenderer.GetPropertyBlock(copiedBlock);