[unity] Added meta file of `add spine-csharp here.txt` to prevent an error message when using Unity Package Manager and git URL.

spine-unity/Assets/Spine/Runtime/spine-csharp/add spine-csharp here.txt.meta

@@ -0,0 +1,7 @@
+fileFormatVersion: 2
+guid: 2b53e3e7c2824094b9259a24cd9a7e03
+TextScriptImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: