shairport-sync: enable Airplay 2 for -openssl only

shairport-sync: enable Airplay 2 for -openssl only

https://github.com/mikebrady/shairport-sync/blob/master/
CONFIGURATION%20FLAGS.md#cryptography states that only the OpenSSL
cryptography backend is suitable for Airplay 2.

Further investigation revealed that the pair_ap module within
shairport-sync, which is needed for Airplay 2, does not have an Mbed TLS
backend.

Accordingly, this commit enables Airplay 2 only for the OpenSSL build.

This has the nice side effect that for Airplay 1 the -mini or -mbedtls
versions can be used without pulling in 6 MB of ffmepg libs.

Signed-off-by: Christian Beier <redacted>