Properties Plugin: Restrict to "gdrive" scheme

No need to load the plug-in in the first place if it is going to
bail out if the item's scheme isn't "gdrive"

src/integration/propertiesplugin/gdrivepropertiesplugin.json

@@ -81,5 +81,6 @@
         "Name[x-test]": "xxGoogle Drive Properties Pagexx",
         "Name[zh_CN]": "Google Drive 属性页面"
     },
-    "MimeType": "inode/directory;application/octet-stream"
+    "MimeType": "inode/directory;application/octet-stream",
+    "X-KDE-Protocol": "gdrive"
 }