Take prefix into account when compressing man pages etc for Flatpak builds
    
    From the original PR (https://src.fedoraproject.org/rpms/rpm/pull-request/15):
    
        The way that a Flatpak works is that the runtime is mounted at /usr
        and the application and included libraries are mounted at /app. So
        the application and included libraries need to be rebuilt with
        a prefix of /app.
    
    This patch makes brp-compress find man pages and info pages under current
    prefix, avoiding the name for spec file changes for the rebuild.