Fix building with webkit < 2.9.2

[ramok]

No description provided.

[keis]

it was added it 2.10 so it shouldn't be added for 2.9 see https://webkitgtk.org/reference/webkit2gtk/stable/WebKitWebsiteDataManager.html#WebKitWebsiteDataManager--disk-cache-directory

we need to do another #if check before adding it to the variables table

[mathstuf]

Upstream never says that symbols are new in x.<odd> releases since they are development versions. You need to look at the actual tarballs to see what releases have which symbols. I'll double check this (I have a git repo with a commit per-release of webkitgtk).