[unity] Fixed build error on Timeline UPM package and Unity 2018+ due to spine-timeline-editor.asmdef being included in normal build.

spine-unity/Modules/com.esotericsoftware.spine.timeline/Editor/spine-timeline-editor.asmdef

@@ -7,7 +7,9 @@
         "Unity.Timeline"
     ],
     "optionalUnityReferences": [],
-    "includePlatforms": [],
+    "includePlatforms": [
+        "Editor"
+    ],
     "excludePlatforms": [],
     "allowUnsafeCode": false,
     "overrideReferences": false,