Ponder adding the version to the FFI header

[rgacogne]

0.22.0 (and some versions before that) broke the API, meaning that downstream users will have to figure out which version of Quiche they are building against at compile-time. At the moment it's a bit painful because it involves testing out the possible signature for various. Defining the current version in quiche.h would make that a lot of easier.
Thanks!