[unity] Fixed compile error on older Unity versions which contain PackageInfo but not FindForAssetPath. See #2223.

spine-unity/Assets/Spine/Editor/spine-unity/Editor/Utility/AssetUtility.cs

@@ -51,7 +51,7 @@
 #define PROBLEMATIC_PACKAGE_ASSET_MODIFICATION
 #endif
 
-#if UNITY_2017_3_OR_NEWER
+#if UNITY_2019_2_OR_NEWER
 #define HAS_PACKAGE_INFO
 #endif