[unity] Fixed compile error due to not all 2020.3 and 2021.1 versions having SaveAssetIfDirty. Closes #2101.

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

@@ -36,7 +36,7 @@
 #define HAS_ON_POSTPROCESS_PREFAB
 #endif
 
-#if UNITY_2020_3_OR_NEWER
+#if (UNITY_2020_3 && UNITY_2020_3_16_OR_NEWER) || UNITY_2021_1_17_OR_NEWER
 #define HAS_SAVE_ASSET_IF_DIRTY
 #endif