Add libtorrent dependency to kiwix-build

[kelson42]

This need to be made available for all kiwix-reader in libkiwix.

This is needed to get kiwix/libkiwix#1251 CI pass.

[kelson42]

@rgaudin @veloman-yunkan One of the three of us can/should to it. In worse case I will try to do it on my own...