fix CMake build with RNP

[dbear496]

Fixes CMake builds when using librnp.

[csoler]

I would be better to have RNP as the default and openpgp-sdk as an option, since RS has officially switched to rnp now.

[G10h4ck]

@csoler @dbear496 to make it default just add a block like this ( in alphabetical order) in topdir CMakeLists.txt

option(
	RS_RNPLIB
	"Use RNP for PGP operations"
	ON )

[dbear496]

Okay, I defined the RS_RNPLIB option in libretroshare with the default ON. However, just a heads up that the default for RS_RNPLIB in retroshare-service is still OFF, so building retroshare-service will still default to not use rnp.

[G10h4ck]

@dbear496 please fix retroshare-service too

[dbear496]

retroshare-service is not part of this repository.

[G10h4ck]

@dbear496 yes, please open a PR on that repository too. @csoler anything else to change before merging?

[csoler]

Does RS_RNP_LIB also works as a preprocessor variable (question from a non cmake user) ? RS needs one to include the proper files.

[dbear496]

@csoler , that's a good catch. I added the USE_OPENPGPSDK define in CMake.