Draft: liblomiri-app-launch: forward declare MirPromptSession, don't include
In this header, we only refer to MirPromptSession as a pointer, so there's no need to include the whole MirClient header into the mix.
This should help building something with both Mir 2 and lomiri-app- launch w/ Mir 1. Although, to be honest, I don't know what might happen at runtime...