libide/webkit: manually enable the favicon database
In WebKitGTK 2.39.6 and 2.39.7, the favicon database is enabled by default. But in all other versions, it is disabled by default.
The new API for enabling the favicon database was added in 2.39.6, so we can use that for the WEBKIT_CHECK_VERSION, even though this code is a no-op before 2.39.8.