rnp_key_add_signature() missing on Ubuntu 22.04

[csoler]

Apparently the version of libRNP that is supplied on ubuntu 22.04 (which is 0.15.2) doesn't include key certification yet.

I could not find in the code a way to prevent the compiler from failing when this functions is not here. Indeed, there is no (apparently) global variable that the pre-compiler can use in an #ifdef statement. I could only find a rnp_version() method, but this cannot be called by the pre-compiler.

Can you suggest me how to proceed? Thx a lot Cyril