Android: pin the rnp crypto backend to openssl for libretroshare too

[jolavillette]

build_librnp builds and installs rnp into the Android sysroot with -DCRYPTO_BACKEND=openssl, but libretroshare's own configure can build rnp a second time, inline: CMakeLists.txt uses add_subdirectory() whenever ../supportlibs/librnp/CMakeLists.txt exists, which is the case as soon as libretroshare is checked out inside the RetroShare super-project rather than standalone.

That inline build gets rnp's default backend, botan, and configure dies with

Could NOT find Botan (missing: BOTAN_LIBRARY BOTAN_INCLUDE_DIR)
(Required is at least version "2.14.0")

since the Android toolchain never builds botan.

Passing the same backend build_librnp uses fixes it. It is a no-op for standalone libretroshare checkouts, where ../supportlibs/librnp does not exist and the pre-built rnp from the sysroot is picked up by find_library() instead — which is why CI never saw this.

Reported on IRC by defnax, who hit it building the AAR for rs-mobile from a super-project checkout.

[jolavillette]

Superseded by #375. Pinning the backend removed the Could NOT find Botan error but only exposed the next one: the inline librnp build also has to run the findopensslfeatures probe it just compiled, which is an Android binary the build host cannot execute. The right fix is not to build librnp inline on Android at all, since the toolchain script already installed one in the sysroot.